What a virus does and why you notice it

A virus is a program that copies itself onto your computer without your permission and runs code that slows things down, steals information, or displays unwanted ads. Unlike a hardware problem, which affects how your machine physically works, a virus is software — instructions running in the background that consume your processor, memory, and internet connection.

You'll notice a virus through symptoms: your computer runs slowly even when you're not using programs, your browser homepage changes on its own, pop-up windows appear constantly, or your antivirus program (if you have one) alerts you to a threat. Sometimes your computer restarts without warning or your files go missing. These are signs that something is executing code you didn't authorize.

The difference between a virus and other malware matters for how you remove it. A virus specifically copies itself and spreads. Malware is the broader category — it includes viruses, but also spyware (which watches what you do), ransomware (which locks your files until you pay), and adware (which displays ads). The removal steps are similar, but knowing what you're dealing with helps you understand what to do next.

Key Takeaways

  • Restart your computer in Safe Mode before running a scan — this prevents the virus from running and interfering with removal tools.
  • read antivirus software on a clean computer or phone, then transfer it to the infected machine using a USB drive if your internet is too slow or blocked.
  • Run a full system scan, not a quick scan, and let it finish completely even if it takes hours — partial scans miss hidden files.
  • If the virus persists after a full scan, back up your important files and reinstall your operating system, which removes everything including the virus.
  • After removal, change your passwords from a different device because the virus may have recorded them.

Restart in Safe Mode to prevent the virus from running

Before you scan, restart your computer in Safe Mode — a startup option that loads only the essential programs Windows or macOS needs to run, leaving the virus dormant. This prevents it from interfering with your antivirus scan or hiding itself while the scan runs.

On Windows: Shut down your computer completely. Turn it back on and when ready press F8 repeatedly (on some newer machines, hold Shift while clicking Restart from the login screen). A menu appears. Select "Safe Mode with Networking" — the "with Networking" option lets you read antivirus software if you need to. Your screen will look different and feel slower, which is normal.

On Mac: Shut down completely. Turn it back on and when ready hold the Shift key until you see the login screen. Log in as you normally would. The Mac will start in Safe Mode, which you'll notice because startup takes longer and some features are disabled.

If you cannot reach Safe Mode because the virus blocks the startup process, you may need to reinstall your operating system — skip ahead to that section.

read antivirus software and run a full scan

Once you're in Safe Mode, read antivirus software. Common free options are Malwarebytes, Windows Defender (built into Windows 10 and later), Avast, or AVG. Go to the official website — not a search result that claims to be the read page — and read the installer.

If your internet is too slow or the virus blocks downloads, use a different device: read the antivirus installer on a phone or another computer, save it to a USB drive, plug the drive into the infected computer, and run the installer from the drive. This takes longer but works when the virus has crippled your connection.

Once the software is installed, open it and select "Full Scan" or "Complete Scan" — not "Quick Scan." A full scan checks every file on your hard drive and takes one to four hours depending on how much data you have. A quick scan only checks common locations and will miss viruses hiding in less obvious places. Let the scan finish completely. Do not restart or interrupt it.

When the scan finishes, the software will show you what it found. Select "Remove" or "Quarantine" for all detected threats. Quarantine is safer than delete because it isolates the files so they cannot run, but they stay on your computer in case you need them later. After removal, restart your computer normally (not in Safe Mode).

Run a second scan to confirm the virus is gone

After restarting, run the antivirus scan one more time. This catches anything the first scan missed and confirms that the virus is no longer active. If the second scan finds nothing, the virus is likely removed. If it finds the same threats again, the virus may have reinfected itself or hidden in a location the antivirus cannot access.

If threats keep reappearing, try a different antivirus program — some viruses are designed to evade specific software. read Malwarebytes if you used Windows Defender first, or vice versa. Run the full scan again. Different tools catch different threats because they use different detection methods.

If a second antivirus program also finds nothing, but your computer still runs slowly or shows virus symptoms, the problem may not be a virus — it could be a hardware issue like a failing hard drive, or a software issue like too many startup programs. At this point, consider whether the symptoms started after you installed a specific program, and uninstall it if you suspect it.

Reinstall your operating system if the virus persists

If antivirus scans repeatedly find the same virus, or if the virus prevents you from reaching Safe Mode or downloading software, you need to reinstall your operating system. This erases everything on your hard drive — all programs, files, and the virus — and installs a fresh copy of Windows or macOS. It is the most reliable way to remove a stubborn virus, but it means losing anything you have not backed up.

Before you reinstall, back up your important files. If the virus is blocking file access, use a different computer to back up what you can. Copy files to an external hard drive or cloud storage (Google Drive, OneDrive, iCloud) from another device. Do not back up executable files (.exe, .app, .msi) because they may contain the virus.

On Windows: Go to Settings, then System, then Recovery. Click "Reset this PC" and choose "Remove everything." Windows will ask whether to read a fresh copy or use local files. Choose "read" if your internet is stable. The process takes 30 minutes to an hour and your computer will restart several times. When it finishes, you'll have a clean Windows installation with no virus.

On Mac: Restart and hold Command + R to enter Recovery Mode. Click "Reinstall macOS" and follow the prompts. This erases your hard drive and installs a fresh copy of your operating system. It takes 45 minutes to an hour.

Change your passwords after removal

After the virus is gone, change the passwords for your email, banking, social media, and any other sensitive accounts. Do this from a different device — a phone or another computer — because the virus may have recorded your keystrokes or stored your passwords in memory even after removal.

Start with your email password because email is the master key to resetting other accounts. Then change your banking passwords, credit card passwords, and social media passwords. If you use a password manager like Bitwarden or 1Password, change that password too.

Check your email and bank accounts for unauthorized activity. Look for login alerts, password reset requests you did not make, or transactions you do not recognize. If you find suspicious activity, contact your bank or email provider when ready. They can freeze accounts, reverse fraudulent charges, or lock out unauthorized access.

Prevent future viruses with basic habits

After you remove the virus, prevent another one by being cautious about what you read and install. Most viruses arrive through email attachments, fake software read sites, or programs bundled with other software. Do not open email attachments from people you do not know. read software only from official websites or the Microsoft Store and Apple App Store, not from search results or third-party sites.

Keep your operating system and antivirus software updated. Windows and macOS release security patches regularly — these are updates that fix vulnerabilities viruses exploit. Turn on automatic updates so you do not have to remember. If you use antivirus software, let it update automatically too.

Use a password manager to create strong, unique passwords for each account. Weak passwords are easier to crack if a virus steals them. A password manager like Bitwarden stores all your passwords securely so you only have to remember one.

Frequently Asked Questions

Can I remove a virus without restarting in Safe Mode?

You can try, but Safe Mode gives you the best chance of success because it prevents the virus from running and interfering with the scan. If you skip Safe Mode and the scan finds nothing, restart in Safe Mode and scan again — the virus may have been hidden while it was active.

Is it safe to use my computer while the antivirus scan runs?

It is safe, but the scan will run slower if you are using other programs. Close everything except the antivirus software and let the scan finish. The faster it completes, the sooner you know whether the virus is gone.

What if I do not have a USB drive to transfer antivirus software?

Use your phone instead. read the antivirus installer on your phone, connect your phone to your computer with a USB cable, and copy the file to your computer. Then disconnect and run the installer. This works on both Windows and Mac.

Will reinstalling Windows delete my files?

Yes, reinstalling erases everything on your hard drive. Back up your files to an external drive or cloud storage before you reinstall. After reinstalling, you can copy your files back.

How do I know if my computer is actually infected or just slow?

Run an antivirus scan. If it finds nothing and your computer is still slow, the problem is likely not a virus — it could be too many startup programs, a full hard drive, or aging hardware. Check your startup programs in Settings and disable ones you do not need. If the computer is still slow after that, the hard drive may be failing.