How to detect intruders on my computer and protect my PC

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The mere thought of someone snooping on your computer without permission is chilling: emails opened, documents reviewed, photos copied, or even remote control of your computer without your knowledge. While it might sound like something out of a hacker movie, it's a much more common situation than it seems, both through online attacks and by people close to you who take advantage of a moment of inattention.

The good news is that Windows, macOS, and your own network offer many clues to help you determine if you have an intruder, and there are tools that can help you confirm suspicions and better protect yourself. In the following lines, you'll see, step by step, how to detect unauthorized access, how to interpret these signs, and what measures to take to secure your PC and Wi-Fi against prying eyes and attackers.

Reasons why someone might want to get into your computer

Putting yourself in the intruder's shoes is key to understanding the risk . Most unauthorized access attempts have one of these intentions: to steal or delete personal data, infect the system with malware, spy on your habits, or use your computer as a gateway for other attackers.

Attackers often rely on Trojans, worms, keyloggers, or remote access tools that install themselves by exploiting vulnerabilities, malicious emails, suspicious downloads, or pirated software. Once inside, they can open "backdoors" to reconnect whenever they want and move around your system almost as if they were sitting right in front of your screen.

In home and work environments, the trusted intruder is also very common : a partner, family member, roommate, or coworker who tries to see your conversations, social media, or personal files when you leave your desk. In these cases, they tend to leave more traces in browsing history, recent documents, or user settings.

Signs that someone might be using your PC

Although remote access can be very stealthy, it's almost never completely invisible . By paying attention to certain system and network behaviors, you can detect early signs that something is wrong.

A clear first symptom is strange network behavior : router lights blinking constantly when you're not downloading anything, high data usage, or an unusually slow connection even with few applications open. If your PC is idle and network activity remains high, it's worth being suspicious.

The hard drive or SSD can also give you away . Constant activity when you don't have any resource-intensive programs open, fans running at full speed, or significantly lower than normal performance can indicate hidden processes running in the background, often linked to malware or remote control tools.

Also, look out for any unusual behavior: the mouse moving on its own , windows or programs appearing without your input, or applications suddenly closing. If you're not handing control over to anyone (for example, a remote support technician), this is a very serious indication that someone is controlling your computer remotely.

Cloud services, email, and apps can also reveal intruders : emails you don't remember sending, strange messages to your contacts, logins from impossible locations or times, or password changes you didn't request. All of this usually points to a compromise of your account or your computer.

Review basic Windows security

Before you get bogged down with advanced scans, make sure your basic protection is active . Windows includes its own antivirus and firewall, which are more than sufficient as a first line of defense if properly configured.

Open Windows Security (shield icon near the clock or by searching for it in Start) and check that all sections are green: Virus & threat protection, Account protection, Firewall & network protection, App and browser control, Device security, etc. If you see any alerts or pending actions, address them immediately.

While Windows Defender is a good starting point, it's not your only option . You can bolster your protection with dedicated malware detection tools like Malwarebytes or similar programs, which are very effective at finding Trojans, worms, keyloggers, and other malware that sometimes slips through the net of your main antivirus software.

For added security, Microsoft offers on-demand scanning utilities such as Microsoft Safety Scanner and the Malicious Software Removal Tool. These are not permanently installed but run periodically to perform a thorough scan and remove persistent threats.

Use of antimalware programs and network tools

When you suspect advanced intruders, it's advisable to combine local protection with network analysis . This way, you not only detect installed malware, but also suspicious connections that might indicate active remote control.

Specialized anti-malware suites are ideal for hunting backdoors . A thorough scan with tools like Malwarebytes can uncover Trojans that open ports, keyloggers that record what you type, or remote access applications disguised as legitimate software.

If you have more advanced technical skills, you can go a step further with port scanners (network analyzers or vulnerability scanners). These programs check which ports on your computer are open and which services are responding, helping you locate suspicious remote services.

These types of tools are typically used to analyze a range of IP addresses , either on your home network or a specific segment, to see if there are exposed devices or devices with ports that shouldn't be accessible. It's a very useful practice for discovering dangerous configurations… provided you know how to interpret the results.

If you're not comfortable with concepts like ports, services, and IP ranges , focus on what you do understand: keeping your antivirus active, running regular full scans, avoiding programs from dubious sources, and monitoring any security alerts issued by these tools.

Event Viewer: Windows' "secret" log

One of the most reliable ways to find out if someone has used your Windows PC is to check the Event Viewer (in professional environments, it's advisable to use monitoring solutions like Wazuh ). This tool records almost everything that happens: logins and logouts, power-ups and shutdowns, system errors, program installations, etc.

To open it, type “Event Viewer” in the Windows search bar and run it. In the left pane, you’ll see several sections: Custom Views, Windows Logs, Application and Services Logs, Subscriptions… The one that’s most relevant to this topic is “Windows Logs.”

Within "Windows Logs" you'll find several categories : Application, Security, Installation, System, and Forwarded Events. To find out if someone has turned on your PC or logged in, focus primarily on "System" and "Security," which is where boot and login/logout data are collected.

The system log shows the exact date and time of each power-on and power-off . Simply compare the recorded events between the time you remember turning off your PC and the next time it was turned on. If there are any intermediate events that you don't recognize, it's very likely that someone used the computer while you were away.

From the Event Viewer's action panel, you can filter by event ID to avoid getting lost in thousands of records. IDs related to logins, shutdowns, and security activity allow you to reconstruct exactly when, how, and from what context the computer was used.

Correlation of events and important IDs

The true power of Event Viewer emerges when you begin correlating IDs , and these logs can be integrated with IDS systems to detect access patterns. Instead of looking at isolated events, you combine them to reconstruct entire usage sessions, which is especially useful when you suspect unauthorized access using your user account.

For example, the combination of events with IDs 4624 and 4647 allows you to clearly see session periods: the first is associated with successful logins, and the second with session logouts. Together, they mark the interval during which someone has been logged in with a specific account.

By carefully reviewing event 4624, you'll uncover much more information than meets the eye : login type (interactive, network, service, batch, etc.), connection source, remote computer name, and so on. It's a goldmine of data for trying to identify the intruder, especially on corporate networks.

For users with a more technical background, it's also worth paying attention to events 4768 and 4771. These are related to the operation of Kerberos, the Windows authentication system, and can be used to identify login attempts with correct or incorrect credentials, as well as suspicious authentication errors.

If you filter the Viewer by these IDs and review them correlated over time , you can find out if someone has tried to log in repeatedly, if they have guessed the password correctly, or if they have even used compromised credentials from another machine on the network.

Modified documents and recent files

Another very practical way to detect snoopers is to check which files have been opened or modified . It won't tell you who did it, but it will tell you if someone has touched documents that shouldn't have been opened.

If you work with Word, Excel, PowerPoint, or other Office applications , you can take advantage of the version history of documents in the cloud (OneDrive, SharePoint, etc.). From the File tab > Info > Version History, you'll see changes and times when the file has been edited.

This history does not directly identify local intruders , but it does reveal if a document has been modified when you were not in front of the PC, or if it has been opened from a different computer that you do not control.

In Windows, you can also use the recent files list as a "silent spy ." By enabling the option to show recently opened items in Start, on the taskbar, and in File Explorer's quick access, you'll have a record of everything that's been opened.

To configure this, go to Settings > Personalization > Start and turn on the "Show recently opened items..." option. From then on, every time you go to Start you'll be able to see which files, folders, or programs have been used recently and detect if there's anything you didn't open.

Browser history and program logs

Your browser is another common telltale sign when someone is snooping around . Checking your browsing history can give you clues if the intruder has visited websites from your user account.

In most browsers (Chrome, Edge, Firefox, etc.), simply press Ctrl + H or click on the three-dot or three-line icon and select "History." There you will see a list, organized by date and time, of the pages visited from that browser profile.

It's true that any reasonably clever intruder will erase their tracks , but they don't always remember everything, or they don't do it well. If you find strange pages, pages with dubious content, or services you haven't used, you already have a clue about unauthorized access.

It's also a good idea to check the cache, search history, and logs of some applications , especially if you use desktop email clients, business chat tools, or work programs that keep logs. Many of these programs store their own history, including timestamps, IP addresses, and accounts used to connect.

Additionally, checking your router's logs or DNS requests leaving your network can help detect unusual browsing activity, even if your browser history has been cleared. To do this, you'll need to access your router's interface and review the connection and device information.

List of installed programs and suspicious applications

One of the first moves of an intruder with serious intentions is to install additional software that makes it easier for them to continue entering or spying better: remote desktop tools, hidden servers, cryptocurrency miners, etc.

That's why it's essential to carefully review the list of installed applications . In Windows, type "Programs" in the Start search bar and go to "Add or Remove Programs" or the Apps section of Settings, depending on your version.

Once there, go through the list, paying close attention to two things : the name/company of each program and the installation date. The most suspicious items are usually among the most recent installations, especially those you don't remember doing.

Don't trust just the icon or the "friendly" name . Some malicious programs try to disguise themselves as system utilities or use names similar to those of well-known manufacturers. If something seems unfamiliar, search for information online and, if you confirm it's junk, uninstall it.

After cleaning up suspicious programs, check the list again after a few days . If any reappear on their own, it's a sign that a malicious component is reinstalling them in the background, and you may need a more thorough cleanup or professional help.

Detect access attempts with Mouse Lock

When you suspect someone nearby is trying to guess your password , it's helpful to know which keys they're trying. For this, there are specific tools like Mouse Lock, which add an extra layer of security to your desktop.

Mouse Lock is a portable software that requires no installation . When run, it displays its own lock screen with a password field separate from the Windows one. You can configure a password and options to keep the pointer within the unlock window or prevent the Task Manager from opening to terminate it.

When you lock your PC with Mouse Lock and someone tries to log in without permission , the program records all the passwords they've entered and the number of failed attempts. When you unlock your computer, you'll see a list of every attempt made.

This log helps you both confirm if someone is actually trying to hack your computer "locally" and assess whether your password is still secure. If you see that they are getting dangerously close to your real password, it's the perfect time to change it to a stronger one.

Other physical clues and additional tricks

It's not all about technical tools; there are also very basic signs worth watching out for . Sometimes, an intruder leaves small traces in the daily use of the computer or peripherals.

The webcam is a good example: pay attention to the camera's LED . If it lights up when you're not on a video call or using any video software, it could indicate that someone has remotely activated the camera to watch or record you.

Another detail to consider is the overall performance of your computer . If it suddenly becomes much slower, the fans spin up rapidly, or CPU and memory usage remains high even when you don't have anything open, something may be running in the background without your knowledge.

Smart plugs can also work in your favor . If you connect your computer to a dedicated smart plug with energy monitoring, the app will show you the times it's been on. While you won't know what the intruder did, you can check if the PC was used when you weren't around.

Finally, check if your browser redirects you to strange websites , full of ads or with suspicious content, or if your search engine and homepage change without your permission. These are very common symptoms of adware or malware, which, besides being annoying, can be the entry point for more serious attacks.

How to protect your account and data in Windows

Once you've confirmed or seriously suspect that someone is using your computer , it's time to put barriers in place to prevent it from happening again. The first step is to ensure that each person has their own password-protected session.

Go to Settings > Accounts in Windows and review the existing users. Make sure your profile is protected with a strong password and, if you share the computer, create separate accounts for each person, preventing everyone from using the same session.

Within "Sign-in options" you'll find several login methods : Windows Hello face, fingerprint, PIN, security key, and classic password. If your computer supports it, combining a PIN or biometrics with a strong account password significantly increases security.

To set or change your classic password, go to the "Password" section , tap "Add" or "Change," enter your new password, confirm it, and add a hint that only you will understand. Avoid birthdates, pet names, or anything easily guessed.

In addition to protecting your Windows session, remember to secure critical applications and services (email, social media, online banking) with unique passwords and, whenever possible, two-step authentication. This way, even if someone manages to get into your PC, it will be much harder for them to access your accounts.

How to tell if someone is using your Mac

If you work with a Mac, you have a very convenient tool to know if the computer has been used even if someone tries to delete histories or superficial traces: the "Screen Time" function.

This feature is available on macOS, iPhone, and iPad and tracks device usage time, which apps were used, and during which times. To view it on your Mac, open System Preferences (or System Settings in recent versions) and search for “Screen Time” in the sidebar or using the magnifying glass.

In this section, you'll see graphs and lists showing your computer's daily activity . If you see usage time during hours you weren't there, or if you see applications open that you don't usually use, it's a pretty clear sign that someone has been tinkering with your Mac.

A key advantage is that "Screen Time" cannot be lightly disabled without leaving a trace, so even if the intruder knows something about computers, they will have a hard time deleting this type of evidence on a daily basis.

Detect and block intruders on your WiFi network

Often the problem isn't just with your PC, but with your home network . If someone infiltrates your Wi-Fi, in addition to hogging your bandwidth, they can try to attack your devices from within or spy on your traffic.

The first thing to look for is the typical signs of Wi-Fi theft : a slower than usual connection, frequent dropouts, router lights constantly flashing, or devices disconnecting for no reason. If all this happens when you're barely using the internet, someone else may be connected.

The most direct way to check this is to access your router's interface . Using a web browser, enter the device's IP address (typical examples are 192.168.1.1 or 192.168.0.1), log in with the administrator username and password (usually found on a sticker underneath the router), and look for a section such as "Connected Devices," "DHCP Clients," or "Network Status."

In that list, you'll see all the devices currently using your Wi-Fi , along with their IP and MAC addresses, and sometimes a descriptive name (mobile, laptop, Smart TV, etc.). Identify which ones are yours, and if you see any unfamiliar devices, they're likely intruders.

If you prefer something more visual, you can use apps like Fing, NetSpot, or Network Analyzer on your mobile device or computer. They scan the network and show you who's connected, the manufacturer of each device, and in some cases, notify you when a new device connects.

Methods to kick intruders out of WiFi

When you confirm that someone else is using your WiFi , you should act as soon as possible to kick them out and prevent them from getting back in.

The most effective step is to change your wireless network password from your router's control panel. Use this opportunity to choose a long password, with uppercase and lowercase letters, numbers, and symbols, that has no direct connection to you or your address.

While you're in the settings, check the security encryption . Ideally, use WPA3; if your router or devices don't support it, use at least WPA2. Avoid WEP or older systems at all costs, as they are very easy to crack.

Another recommended setting is to disable WPS (the physical button or the quick Wi-Fi setup option). While convenient for connecting devices, it's also a common entry point for attackers trying to force the WPS PIN.

If you want to take it a step further, configure a "whitelist" of devices using MAC filtering, allowing only the MAC addresses of your own devices. It's not foolproof (MAC addresses can be spoofed), but it adds an extra layer of protection for less advanced users.

If after all this you still see strange devices or the network behaves strangely, contact your operator to have the line checked and, if necessary, replace the router or help you configure additional measures.

Detecting and stopping intruders on your computer and network is a matter of combining common sense, a bit of technical curiosity, and some best practices: watch for unusual activity, review logs such as the Event Viewer or "Usage Time," keep your antivirus software and operating systems up to date, protect your passwords, and don't neglect your Wi-Fi. If you make these habits a routine, it will be much harder for someone to infiltrate your PC without you noticing.