How to scan your computer for viruses
The fastest way to check for viruses is to run a full scan using the antivirus software already built into your operating system. On Windows, that's Windows Defender (called Microsoft Defender on newer versions). On Mac, the system includes XProtect, which runs automatically in the background. Both scan your files and running programs without you paying anything extra.
To run a manual scan on Windows, open Settings, go to Privacy & Security, then Windows Security. Click Virus & threat protection, then Scan options. Select Full scan and click Scan now. A full scan takes 30 minutes to several hours depending on how many files you have. You can use your computer while it runs, though it will be slower.
On Mac, open System Settings, go to General, then Security & Privacy. You won't see a manual scan button—XProtect runs continuously. If it detects something, it will notify you and ask what to do. You can also run a scan using the built-in Malware Removal Tool by restarting your Mac and holding Command+Shift+U during startup, though this is less common and usually only needed if you suspect a serious infection.
Key Takeaways
- Windows Defender and Mac's XProtect are free and built in—you do not need to buy separate antivirus software to check for viruses.
- A full scan on Windows takes 30 minutes to several hours and checks every file on your computer, not just recently downloaded ones.
- If a scan finds something, the software will quarantine it (move it to a safe folder where it cannot run) and ask you to remove it.
- Running a scan once a month or after downloading files from untrusted sources is a reasonable routine; you do not need to scan constantly.
- If your computer is running very slowly or behaving strangely, a full scan is the first troubleshooting step before calling for help.
What happens when a virus is found
When your antivirus software detects a virus or malware, it does not automatically delete it. Instead, it quarantines the file—moves it to an isolated folder where it cannot run or spread. The software then shows you a notification asking what you want to do: remove it, restore it, or ignore it.
In most cases, you should click Remove or Delete. The software will delete the quarantined file permanently. If the scan found something in a program you use regularly and you are not sure whether it is actually a threat, you can search the file name online to see whether other people have reported it as malware. Legitimate programs sometimes trigger false alarms, though this is rare with Windows Defender and XProtect.
If the scan finds multiple threats, remove all of them. If it finds something in a system file (a file that Windows or macOS needs to run), the software may ask for permission to restart your computer to complete the removal. Let it restart—this is normal and safe.
Scanning with a second antivirus tool
If you want a second opinion or suspect your computer has a serious infection that the built-in scanner missed, you can run a scan with a different antivirus program. Malwarebytes and Kaspersky Rescue Disk are two options that many people use alongside Windows Defender or XProtect. Both offer free versions that scan your computer without installing permanent software.
Malwarebytes can be downloaded and run once, then uninstalled. Kaspersky Rescue Disk creates a bootable USB drive that scans your computer before Windows or macOS even starts up, which can catch infections that hide from the normal operating system. These tools are useful if your computer is behaving very strangely or if you think you have been targeted specifically, but they are not necessary for routine checking.
Do not install multiple antivirus programs permanently on the same computer. They will conflict with each other and slow your system down. Use the built-in scanner as your main tool, and run a second scanner only when you suspect a problem.
Signs your computer might have a virus
A virus or malware infection often shows itself through changes in how your computer behaves. Your computer might run much slower than usual, even when you are not using demanding programs. Programs might crash or freeze. Your browser might redirect you to websites you did not click on, or show pop-up ads constantly. Your antivirus software might stop working or turn itself off.
You might also notice your hard drive light blinking constantly, or your computer making noise from the fan running at full speed—signs that something is using a lot of processing power in the background. Files might go missing or change names. Your internet connection might be slow even though your router is working fine.
None of these signs always means you have a virus—they can also mean your hard drive is failing, your computer is overheating, or you straightforward have too many programs running. But they are all reasons to run a full scan and see what the antivirus software finds.
Preventing viruses before they get on your computer
The best protection is not letting viruses onto your computer in the first place. Do not read files from websites you do not recognize or trust. Be cautious with email attachments, especially from people you do not know—malware is often hidden in Word documents, PDFs, or ZIP files. If someone sends you an unexpected attachment, ask them to confirm they meant to send it before you open it.
Keep your operating system and all your programs updated. Windows and macOS release security updates regularly that patch holes that viruses exploit. Turn on automatic updates so you do not have to remember to do it manually. Updates sometimes require a restart, but they are worth the inconvenience.
Use a password manager to create strong, unique passwords for each website you use. If one website is hacked, a strong unique password means the hacker cannot use that password to break into your other accounts. Enable two-factor authentication on important accounts like email and banking—this means even if someone steals your password, they cannot log in without a code from your phone.
When to call a professional
If you run a full scan and the antivirus software finds nothing, but your computer is still behaving strangely, the problem might not be a virus. It could be a hardware failure, a corrupted operating system file, or a program conflict. At that point, calling a local computer repair shop or your computer manufacturer's support line is the right move.
If a scan finds a virus but the antivirus software cannot remove it, or if your computer will not start up at all, you also need professional help. A technician can boot your computer from a USB drive or take it apart to clean the infection from outside the normal operating system.
If you think you have been the target of a targeted attack (for example, you work in a sensitive field or received a threatening email), contact your IT department if you have one, or a cybersecurity professional. These situations are rare for home computer users, but they need more than a standard antivirus scan.
Frequently Asked Questions
How often should I scan my computer for viruses?
Running a full scan once a month is reasonable for most people. If you read files frequently from the internet or visit risky websites, scan every two weeks. If you mostly use your computer for email and web browsing from trusted sites, once a month is enough. You do not need to scan every day.
Is Windows Defender or Mac's XProtect really enough, or do I need to buy antivirus software?
The built-in scanners are genuinely effective for most home users. They catch the vast majority of common viruses and malware. You do not need to buy a paid antivirus program unless you have specific reasons—for example, if you run a business and handle sensitive customer data, or if you have been infected before and want extra layers of protection.
What if the antivirus software says a file is a virus but I know it is a legitimate program?
This is called a false positive. Search the file name online to see whether other people have reported it as safe. If it is a well-known program from a major company, it is almost certainly safe and the antivirus is being overly cautious. You can tell the antivirus to ignore it or restore it from quarantine. If you are unsure, leave it quarantined and do not restore it.
Can I get a virus from visiting a website?
Yes, but it is rare with modern browsers and operating systems. Visiting a malicious website might try to read malware to your computer, but Windows Defender and XProtect will usually block it. The bigger risk is downloading a file from a website and then opening it. Do not read files unless you are sure the website is legitimate.
Does a virus scan use a lot of internet data?
No. A scan checks the files already on your computer, not files on the internet. It uses almost no internet data. You can run a scan on a metered connection (like a phone hotspot) without worrying about using up your data limit.