Where to start: the built-in Windows or Mac scanner

Your computer already has a virus scanner built in. On Windows, it's called Windows Defender (or Microsoft Defender on newer versions). On Mac, the system runs background checks automatically, though you can also use Malwarebytes or a similar tool. Starting with what you already have costs nothing and requires no installation.

To run a scan on Windows, open the Start menu, type "Windows Security," and click it. Select "Virus & threat protection" on the left side. You'll see a button that says "Scan options." Click it, choose "Full scan," and then click "Scan now." A full scan takes 30 minutes to several hours depending on how much is on your drive, so start it when you won't need the computer.

On Mac, open System Preferences, go to Security & Privacy, and check the "General" tab. If you see a message about a virus or malware, follow the on-screen steps. If nothing appears, your system has not detected anything recent. You can also manually run a scan using the Malwarebytes app if you install it.

Key Takeaways

  • Windows Defender and Mac's built-in protection are your first line of defense and work without extra cost or installation.
  • A full scan checks every file on your computer and takes several hours, while a quick scan checks only the most common infection spots and takes minutes.
  • If a scan finds something, the scanner will quarantine it (move it to a safe holding area) rather than delete it when ready, so you can review what was found.
  • Running a second scan with a different tool like Malwarebytes can catch threats the first scanner missed, because different tools use different detection methods.
  • Restart your computer in Safe Mode before scanning if you suspect an active infection, because some viruses stop the scanner from running properly.

Understanding scan types and what each one does

When you open the scan options, you'll see choices: Quick scan, Full scan, and sometimes Custom scan. A Quick scan checks only the folders where viruses most often hide — your Downloads folder, temporary files, and system startup locations. It takes 5 to 15 minutes. A Full scan checks every single file on your entire drive. It's slower but more thorough.

Use a Quick scan first if you're in a hurry or just want a routine check. Use a Full scan if you think something is actually wrong — your computer is slow, programs are crashing, or you clicked a suspicious link. A Custom scan lets you pick specific folders to check, which is useful if you only want to scan your Downloads folder or an external drive.

What the scan results actually mean

When a scan finishes, it shows you what it found. The scanner will display a number of threats detected, or it will say "No threats found." If threats appear, the scanner moves them to Quarantine, which is a locked folder where they can't run or spread. Quarantine is not deletion — the files are still there, just isolated.

Read the list of what was found. Most of the time, the scanner caught something harmless or something you already knew about. If you see a file you recognize — like an installer you downloaded — you can review it before the scanner removes it. If you see something you don't recognize at all, it's safer to let the scanner delete it. Click "Remove" or "Delete" to finish.

If the scan finds nothing, that's good news, but it doesn't mean your computer is completely safe. Scanners can only catch viruses they know about. New threats emerge constantly, and some sophisticated malware hides in ways that current scanners might miss.

When to use a second scanner for a second opinion

Different antivirus tools use different methods to spot threats. Windows Defender is reliable for everyday protection, but running a second scan with a different tool can catch things the first one missed. Malwarebytes is a common second choice — it's free to read and run a single scan, though the full version costs money.

read Malwarebytes from its official website (malwarebytes.com), install it, and run a scan. Don't install two antivirus programs to run at the same time — they interfere with each other. But running one scan with Windows Defender and then one with Malwarebytes, one after the other, is safe and gives you better coverage.

If both scanners find nothing, your computer is almost certainly clean. If one finds something the other missed, that's useful information. Review what was found and decide whether to remove it based on whether you recognize the file.

Scanning external drives and USB sticks

Viruses can hide on external drives and USB sticks just as easily as on your main computer. Plug in the external drive, open Windows Security or your scanner, and choose Custom scan. Select the external drive from the list of locations and run the scan. This takes longer than scanning a folder because external drives are often slower, but it's important if you use the drive on multiple computers.

If a scan finds threats on an external drive, quarantine or remove them the same way you would on your main computer. After that, the drive is safer to use. If you're moving files between computers, scanning the drive before you plug it into a new machine is a good habit.

Safe Mode scanning for stubborn infections

If your computer is running very slowly, programs keep crashing, or you suspect an active infection, restart in Safe Mode before scanning. Safe Mode loads only the essential parts of Windows, which prevents most viruses from running. This gives the scanner a better chance of finding and removing them.

On Windows 10 or 11, hold down the Shift key and click the power button in the Start menu, then select "Restart." When the computer restarts, you'll see a menu. Click "Troubleshoot," then "Advanced options," then "Startup Settings," and finally "Restart." When the menu appears again, press 4 or F4 to start in Safe Mode. Your screen will look different and fewer programs will load, but this is normal.

Once in Safe Mode, open Windows Security and run a Full scan. The scan will take longer because Safe Mode is slower, but it will catch infections that hide from the scanner in normal mode. After the scan finishes and threats are removed, restart normally.

What to do if the scanner won't run or keeps crashing

Sometimes a virus is aggressive enough to block the scanner from running at all. If Windows Security won't open, or it opens but the scan button doesn't work, try these steps in order: First, restart your computer. Second, try Safe Mode (see the section above). Third, read the Windows Defender Offline scan from Microsoft's website on a different computer, put it on a USB stick, and run it from the USB stick on the infected computer.

If none of that works, or if you're not comfortable doing it yourself, take the computer to a repair shop. They have specialized tools and can remove stubborn infections. This costs money, but it's worth it if the infection is serious enough to block your own scanner.

Frequently Asked Questions

How often should I scan my computer for viruses?

Run a Quick scan once a week or once a month as routine maintenance. Run a Full scan if you think something is wrong, or if you've been to risky websites or opened suspicious email attachments. Most people don't need to scan more than monthly if they have antivirus software running in the background.

Can I use my computer while a scan is running?

Yes, but the scan will run much slower and your computer will be sluggish. It's better to start a Full scan and then leave the computer alone for a few hours. Quick scans are fast enough that you can use the computer at the same time without much slowdown.

What if the scanner says it found a virus but won't remove it?

Some viruses lock themselves so the scanner can't delete them while Windows is running. Restart in Safe Mode and scan again — Safe Mode gives the scanner more power to remove stubborn threats. If it still won't remove, restart in Safe Mode with Command Prompt and let the scanner try once more.

Is it safe to delete quarantined files?

Yes. Quarantine means the files are already isolated and can't hurt anything. If you're sure you don't need them, you can delete them permanently. If you're unsure, you can leave them in quarantine for a while and delete them later if nothing breaks.

Do I need to pay for antivirus software?

No. Windows Defender is free and built in, and Malwarebytes offers a free scan. Paid antivirus software offers extra features like real-time protection and technical support, but for basic scanning and protection, the free tools are enough for most people.