Signs Your Computer Might Be Infected

A virus on your computer usually shows itself through changes in how your machine behaves. The most common signs are a computer that runs much slower than normal, programs that crash or freeze without reason, and a fan that runs constantly even when you are not using demanding software. You might also notice your internet connection dropping repeatedly, or your browser opening pages you did not request.

Other warning signs include unexpected pop-up windows that appear even when you are not browsing the web, files or folders that disappear or change names on their own, and a hard drive that seems to be working (you hear clicking or see the light blinking) even when you are not doing anything. Some viruses also cause your computer to restart on its own, or display messages asking you to pay money or read something.

Keep in mind that slow performance and crashes can also happen for reasons that have nothing to do with viruses — a full hard drive, outdated software, or a failing hard drive can cause the same symptoms. This is why checking for a virus is one step in troubleshooting, not the only step.

Key Takeaways

  • Slow performance, unexpected pop-ups, and constant fan noise are common signs a virus may be present, but they can also result from a full hard drive or outdated software.
  • Windows Defender (built into Windows) and Malwarebytes are two widely used tools that scan your computer for viruses and malware without costing money.
  • Before running a scan, restart your computer in Safe Mode, which loads only essential programs and makes it harder for a virus to interfere with the scan.
  • If a scan finds a virus, the scanning tool will usually offer to remove it automatically; if removal fails, you may need to reinstall Windows or contact a repair technician.
  • Keeping your operating system and software updated, using a password manager, and avoiding suspicious email links and downloads prevent most virus infections.

How to Scan Your Computer for Viruses

Windows Defender is built into every Windows computer and runs in the background automatically. To run a full scan manually, click the Windows Start button, type "Windows Defender" and open the app. Select "Scan options" and choose "Full scan," then click "Scan now." A full scan can take 30 minutes to several hours depending on how much data is on your computer and how many files are infected.

If you want a second opinion or prefer a different tool, Malwarebytes is free to read and use for scanning. Go to malwarebytes.com, read the free version, install it, and open the program. Click "Scan" and let it run. Malwarebytes scans faster than Windows Defender but may not catch every type of threat, which is why running both is a reasonable approach.

Do not run multiple full scans at the same time — this slows your computer and can cause the scans to interfere with each other. Run one scan, wait for it to finish, then run the other if you want to.

Restarting in Safe Mode Before You Scan

Safe Mode is a Windows feature that starts your computer with only the bare minimum programs running. This matters because some viruses are designed to stop antivirus software from working, and Safe Mode makes that harder for them to do. Restarting in Safe Mode before you scan gives the scan the best chance of finding and removing threats.

To restart in Safe Mode on Windows 10 or 11, click the Start button, then the power icon, then hold Shift and click "Restart." Your computer will show a menu with options — select "Troubleshoot," then "Advanced options," then "Startup Settings," then "Restart." When your computer restarts, press the number 4 or F4 to enter Safe Mode. The screen will look different and fewer programs will be running. Once you are in Safe Mode, open Windows Defender or Malwarebytes and run your scan.

After the scan finishes and you have dealt with any threats it found, restart your computer normally by clicking Start, the power icon, and "Restart." Your computer will return to its normal state.

What to Do If a Virus Is Found

When Windows Defender or Malwarebytes finds a virus, the program will show you what it found and ask what you want to do. In most cases, you should click the button to remove or quarantine the threat — quarantine means the program locks the infected file away so it cannot run, which is usually safer than deleting it when ready in case the program made a mistake.

If the scanning tool says it cannot remove a threat, the virus may be deeply embedded in your system. In that case, you have two main options: reinstall Windows (which erases everything on your computer but removes the virus completely), or take your computer to a repair technician who has tools and experience for stubborn infections. Reinstalling Windows is free but time-consuming; a repair shop usually costs money but takes less of your time.

After removal, restart your computer and run the scan again to make sure the threat is gone. Some viruses leave pieces behind that need a second scan to catch.

Preventing Viruses in the First Place

The easiest way to avoid a virus is to not let one onto your computer. Keep Windows and all your software updated — updates often patch security holes that viruses use to get in. Turn on automatic updates in Windows settings so you do not have to remember to do this yourself.

Be cautious with email attachments and links, especially from people you do not know. Do not read files from websites that look suspicious or ask you to disable your antivirus software. If a website or email is asking you to read something to "fix" your computer or "claim a prize," it is almost certainly a scam.

Use a password manager like Bitwarden or 1Password to create strong, unique passwords for each website you use. This way, if one website is hacked, the attacker cannot use that password to break into your other accounts. Avoid reusing the same password across multiple sites, as this is how viruses and hackers spread from one account to another.

When to Seek Professional Help

If you have run a full scan in Safe Mode, removed all threats the scanner found, and your computer still behaves strangely, the problem may not be a virus. It could be a hardware failure, a corrupted operating system, or a software conflict that a scan cannot fix. At this point, taking your computer to a repair shop makes sense.

You should also seek help if you are not comfortable restarting in Safe Mode or running a scan yourself. A technician can do this work for you and explain what they found. Many repair shops offer free diagnostics, so you can find out what is wrong before you decide whether to pay for a fix.

Frequently Asked Questions

Can a virus spread from my computer to my phone or tablet?

Most viruses written for Windows computers cannot run on phones or tablets because they use different operating systems. However, if you use the same password on multiple devices, a hacker who steals it from your computer can break into your phone accounts. Use a password manager to keep passwords unique across all your devices.

Is it safe to use my computer while a virus scan is running?

You can use your computer during a scan, but it will be slow because the scan is using your processor and hard drive. It is better to start a scan and then step away, or run it overnight. Do not restart your computer while a scan is in progress, as this can interrupt it.

What is the difference between a virus and malware?

A virus is a type of malware — malware is the broad category that includes viruses, spyware, ransomware, and other harmful software. Windows Defender and Malwarebytes both scan for all types of malware, not just viruses, so the distinction does not matter much for your purposes.

Do I need to pay for antivirus software?

No. Windows Defender is free and built in, and Malwarebytes has a free version that scans and removes threats. Paid antivirus software offers extra features like real-time monitoring and customer support, but for most people the free tools are enough.

Can I catch a virus just by visiting a website?

It is possible but uncommon. Most viruses require you to read and run a file, or click a link in an email. Visiting a website alone is usually safe unless the site is actively trying to exploit a security hole in your browser — keeping your browser and Windows updated makes this very unlikely.