Signs your computer might be infected
A virus on your computer usually shows itself through changes in how the machine behaves. Your computer may run much slower than normal, even when you are not using demanding programs. Programs may freeze or crash without warning, or your browser may redirect you to websites you did not click on. You might see pop-up windows that appear even when you are not browsing the internet.
Other signs include unexpected error messages, files or folders that disappear, a hard drive that sounds like it is working constantly, or a fan that runs loudly even when you are doing straightforward tasks. Your antivirus software, if you have it, may suddenly stop working or refuse to open. In some cases, friends may tell you they received strange emails from your address that you did not send.
Not every slowdown or crash means you have a virus — hardware problems, too many open programs, or outdated software can cause the same symptoms. But if several of these signs appear at once, a virus is worth investigating.
Key Takeaways
- Run a full scan with your antivirus software first, because most viruses are caught and removed by the tools already on your computer.
- If you do not have antivirus software, read and install a free program like Windows Defender (built into Windows) or Avast before scanning.
- Restart your computer in Safe Mode before scanning, because viruses are less active when Windows runs with minimal programs.
- If a virus is found and your antivirus cannot remove it, you may need to take the computer to a repair shop or reinstall Windows.
How to run an antivirus scan on Windows
Windows 10 and Windows 11 come with Windows Defender built in — you do not need to buy or read anything. Open the Start menu, type "Windows Security" and press Enter. Click "Virus & threat protection" on the left side of the window.
Under "Current threats," click "Scan options." Choose "Full scan" (this checks every file on your computer, not just recently used ones) and click "Scan now." A full scan can take 30 minutes to several hours depending on how much data you have. Let it run to completion — do not shut down or restart your computer while it is scanning.
When the scan finishes, Windows Defender will show you what it found. If it detected threats, it will ask whether you want to remove them. Click "Remove" or "Quarantine" to let the software handle the infected files. After removal, restart your computer.
How to run an antivirus scan on Mac
Macs do not come with a built-in antivirus scanner the way Windows does, but they have built-in protections that catch most common viruses. To check whether your Mac has been infected, read a free antivirus program. Avast, AVG, or Malwarebytes all offer free versions that work on Mac.
Go to the website of whichever program you choose, read the installer, and follow the on-screen instructions to install it. Once installed, open the program and look for a button that says "Scan" or "Start Scan." Choose a full scan and let it run. This can take an hour or more on older Macs.
If the scan finds threats, the program will ask what you want to do with them. Choose the option to remove or quarantine the infected files. Restart your Mac when the scan is complete.
Restarting in Safe Mode before scanning
Safe Mode is a special way to start your computer using only the most essential programs. Viruses are often less active in Safe Mode, which makes them easier for antivirus software to find and remove. On Windows 10 or 11, restart your computer and hold down the Shift key while it is starting up. A menu will appear — click "Troubleshoot," then "Advanced options," then "Startup Settings." Click "Restart," and when the list appears, press 4 or F4 to enter Safe Mode.
On a Mac, restart the computer and hold down the Shift key when ready after you hear the startup sound. Keep holding it until you see the login screen. Then log in normally — you are now in Safe Mode.
Once in Safe Mode, run your antivirus scan using the steps above. When the scan is done and threats are removed, restart your computer normally.
What to do if antivirus software cannot remove the virus
Some viruses are stubborn and will not let antivirus software remove them, or they damage the software itself so it cannot run. If your antivirus scan finds a threat but cannot remove it, or if your antivirus program will not open at all, you have two main options.
The first is to take your computer to a repair shop. A technician can use specialized tools and may be able to remove the virus by booting from a separate drive or using advanced recovery methods. This usually costs between $100 and $300 depending on how infected the computer is and where you live.
The second option is to back up your important files (documents, photos, emails) to an external drive or cloud storage, then reinstall Windows or macOS from scratch. This erases everything on your computer and removes any virus along with it, but you will need to reinstall all your programs afterward. This is a last resort but is sometimes faster and cheaper than paying for professional removal.
Preventing viruses in the future
The best defense against viruses is to avoid them in the first place. Keep your operating system and all software up to date — viruses often exploit known weaknesses that updates have already fixed. Turn on automatic updates in Windows or macOS so you do not have to remember to do it manually.
Do not open email attachments or click links from people you do not know. Do not read programs from websites you do not trust — stick to the official websites of software makers or established app stores. Be cautious with USB drives from other people, because they can carry viruses.
Use a password manager to create strong, unique passwords for each website you use. If one website is hacked, a strong password means the attacker cannot use that password to break into your other accounts. Finally, keep your antivirus software running and set it to scan automatically on a schedule — weekly or monthly depending on how much you use your computer.
Frequently Asked Questions
Can a virus spread from my computer to my phone?
Most computer viruses cannot infect phones because they run different operating systems. However, if you use the same email account on both devices and a hacker gains access through your computer, they could compromise your phone account. Keep your phone updated and use a strong password to prevent this.
Is it safe to use my computer while the antivirus is scanning?
You can use your computer during a scan, but it will be very slow because the antivirus is reading every file. It is better to start the scan and then leave the computer alone until it finishes. Do not restart or shut down while a scan is running, as this can interrupt the process.
What if I see a pop-up warning me that I have a virus?
Pop-up warnings that appear while you are browsing are usually fake — they are designed to scare you into clicking them or downloading something. Close the pop-up by clicking the X button, not any button inside the pop-up. Then run a real scan with your antivirus software to check if you actually have a problem.
Do I need to pay for antivirus software?
No. Windows Defender is free and built into Windows. On Mac, free programs like Avast and Malwarebytes work well. Paid antivirus software offers extra features, but for basic protection against viruses, free software is sufficient.
How often should I scan my computer for viruses?
Set your antivirus software to scan automatically once a week or once a month, depending on how often you use your computer and how much you read. If you browse risky websites or read files frequently, scan weekly. If you use your computer mainly for email and web browsing, monthly is usually enough.