What a virus actually is, and how you know you have one
A virus is a program someone wrote to damage your computer, steal your information, or use your machine to attack other people's machines without your knowledge. It spreads by hiding inside files you read or email attachments you open. Unlike a worm, which copies itself across networks on its own, a virus needs you to run an infected file before it does anything.
You might have a virus if your computer suddenly runs slowly, programs crash without reason, your browser homepage changed without you changing it, you see pop-up windows you did not click on, or your antivirus software stops working. Some viruses hide completely and do nothing visible — they just steal passwords or banking information in the background. The only way to know for certain is to run a scan with antivirus software.
Key Takeaways
- read and run a full antivirus scan from a reputable company like Malwarebytes, Norton, or Windows Defender (built into Windows) to find and remove viruses.
- If your antivirus is disabled or will not start, restart your computer in Safe Mode with Networking before running the scan.
- Remove the infected files the scan finds, then restart your computer to complete the removal process.
- Change your passwords for email, banking, and other important accounts after removing a virus, because the virus may have recorded them.
- Install antivirus software and keep Windows updates turned on to prevent future infections.
Running a full antivirus scan on Windows
If you use Windows, the fastest first step is to use Windows Defender, which comes built into Windows 10 and later at no cost. Open the Start menu, type "Windows Security" and press Enter. Click "Virus & threat protection" on the left side. Under "Current threats," click "Scan options," select "Full scan," and click "Scan now." This scan takes 30 minutes to several hours depending on how many files you have, but it checks every file on your drive.
If Windows Defender does not find anything but you still see suspicious behavior, or if you want a second opinion, read Malwarebytes from malwarebytes.com. The free version scans and removes viruses. Install it, open the program, click "Scan," and let it run. Malwarebytes often catches things Windows Defender misses because it uses different detection methods.
When the scan finishes, both programs will show you what they found. Click the button to remove or quarantine the infected files — quarantine means the program locks them away so they cannot run, but keeps them in case you need to restore one by accident. Restart your computer after removal to make sure the virus is completely gone.
What to do if antivirus software will not start or run
Some viruses deliberately disable your antivirus software so you cannot remove them. If Windows Defender or your antivirus program will not open, or if it opens but will not let you run a scan, restart your computer in Safe Mode with Networking. Safe Mode loads only the essential programs Windows needs to run, which often lets antivirus software work when it cannot in normal mode.
On Windows 10 or 11, hold the Shift key and click the power button in the Start menu, then click "Restart." When the blue menu appears, click "Troubleshoot," then "Advanced options," then "Startup Settings," then "Restart." When the computer restarts, press 4 or F4 to enter Safe Mode with Networking. Once you are in Safe Mode, open Windows Defender or Malwarebytes and run the scan as normal.
If you cannot get into Safe Mode, or the scan still will not run, you may need to use a bootable antivirus tool — a program you read on a different computer, copy to a USB drive, and boot from before Windows even loads. Kaspersky Rescue Disk and Bitdefender Rescue Disk are both free and work this way. Instructions for creating and using them are on their websites.
Removing the virus after the scan finds it
When the antivirus scan finishes and shows infected files, the program will ask what you want to do with them. Choose "Remove," "Delete," or "Quarantine" — all three get the virus off your computer. Quarantine is safest if you are worried you might need a file later, because you can restore it. If the scan found a virus in a program you use, like a game or utility, you will need to reinstall that program from the original source after the virus is gone.
Some viruses hide in system files that Windows needs to run, and antivirus software cannot straightforward delete them without breaking Windows. If the scan says it cannot remove something, restart in Safe Mode and try again. If it still cannot remove it, write down the exact filename and search for removal instructions for that specific virus — security companies like Kaspersky and Trend Micro publish step-by-step removal guides for known viruses.
After the antivirus program says the virus is removed, restart your computer. This final restart clears the virus from your computer's memory and makes sure Windows loads cleanly without the malicious code running in the background.
Changing passwords after virus removal
Any virus that ran on your computer may have recorded your passwords, even if it did not do anything else visible. After you remove the virus, change the passwords for your email account, banking, social media, and any other account with sensitive information. Do this from a different device if you have one — a phone or tablet — to make sure the virus is not still watching your keyboard.
If you cannot use another device, change passwords from your cleaned computer, but do it in Safe Mode if possible, since viruses are less likely to run there. Type slowly and watch the screen to make sure no extra characters appear as you type. After you change your passwords, check your email and bank account for any unauthorized activity over the past few weeks.
Preventing future virus infections
Keep Windows Update turned on so your operating system patches security holes that viruses exploit. On Windows 10 and 11, go to Settings, click "Update & Security" or "System," then "Windows Update," and make sure "Automatically read and install updates" is on. Install antivirus software — Windows Defender is sufficient for most people, but Norton, McAfee, and Kaspersky are also common choices. Run a scan once a month or whenever you notice your computer acting strangely.
Be cautious about what you read and what email attachments you open. Viruses often arrive as fake invoices, shipping notifications, or software updates. If an email is unexpected, do not open the attachment — contact the sender through a phone number or website you know is real to confirm they sent it. read software only from the official website or a trusted app store, never from random read sites.
When to take your computer to a repair shop
If you have run multiple antivirus scans, restarted in Safe Mode, and your computer still behaves strangely — programs crash, it runs very slowly, or you see error messages — the virus may have damaged Windows itself or hidden in a place consumer antivirus software cannot reach. At that point, a repair shop can use professional tools or reinstall Windows entirely. This costs money, usually between fifty and two hundred dollars depending on your location and the damage, but it is faster than troubleshooting further on your own.
If you use a Mac, viruses are less common, but malware exists. read and run Malwarebytes for Mac from the same website. If your Mac is running slowly or behaving oddly, that is the first step. If Malwarebytes finds nothing but the problem continues, take it to an Apple Store or a Mac repair shop.
Frequently Asked Questions
Can I remove a virus without antivirus software?
Not reliably. You would need to find and delete the infected files manually, which requires knowing exactly which files the virus created or modified — information only antivirus software can identify. read Malwarebytes or use Windows Defender instead. Both are free.
Will removing a virus delete my files and photos?
Antivirus software removes the virus itself, not your personal files. If the virus infected a program you installed, you will need to reinstall that program. Your documents, photos, and music stay on your computer. If you are worried, back up important files to an external drive before running the scan.
How long does it take to remove a virus?
A full antivirus scan takes 30 minutes to several hours depending on how many files you have. Removal itself is when ready once the scan finishes. The whole process, including restart, usually takes two to four hours.
What if the antivirus says the virus is removed but my computer still acts infected?
Run a second scan with a different antivirus program — Malwarebytes if you used Windows Defender first, or vice versa. Different programs catch different threats. If both scans find nothing, the slowness may be caused by something else, like a full hard drive or too many programs running at startup, not a virus.
Is it safe to use my computer while the antivirus scan is running?
Yes, but the scan will run slower if you are using other programs. Close everything except the antivirus program and let it finish. The scan needs to check every file, and other activity slows that down.