What scanning for viruses actually does

A virus scan searches your hard drive, memory, and running programs for code patterns that match known malware. The scan compares files on your computer against a database of signatures — digital fingerprints of viruses, spyware, and other unwanted software. When it finds a match, it alerts you and can quarantine or remove the threat.

Scanning does not prevent infection in the first place. It finds what is already there. A good antivirus program runs in the background to block threats before they land, but scanning is how you check whether something got through.

Most computers come with built-in scanning tools. Windows includes Windows Defender, and macOS includes XProtect. You can also install third-party antivirus software like Malwarebytes, Norton, or Kaspersky. Each works the same way: it scans, detects, and removes.

Key Takeaways

  • Windows Defender and macOS's built-in tools are free and run automatically, so you may already have protection without installing anything.
  • A full system scan takes 30 minutes to several hours depending on how much data you have, so start it when you do not need the computer.
  • If a scan finds threats, the antivirus program will ask whether to quarantine or remove them — quarantine is safer if you are unsure.
  • Scanning once a week or after visiting unfamiliar websites catches most problems early, before they slow your computer or steal data.

How to run a scan on Windows

Open Windows Defender by typing "Windows Defender" into the search box at the bottom left of your screen and clicking the app. Once it opens, click Virus & threat protection on the left side.

You will see three scan options: Quick scan, Full scan, and Custom scan. A quick scan checks the most common places where malware hides — your Downloads folder, temporary files, and running programs — and takes about 5 to 10 minutes. A full scan checks every file on your entire hard drive and takes 30 minutes to several hours. Start with a quick scan; if it finds nothing and you are concerned, run a full scan.

Click the scan type you want, then click Scan now. Your computer will continue to work while scanning, though it may run slower. When the scan finishes, Windows Defender will show you what it found. If it found threats, click Actions and choose whether to quarantine or remove them.

How to run a scan on Mac

macOS runs virus protection in the background automatically through XProtect, so you do not need to open anything to be scanned. However, you can manually check the status by opening System Settings, clicking General, then Security & Privacy. Look for any alerts about detected threats.

If you want more detailed scanning, read Malwarebytes for Mac (free version available). Open the app, click Scan, and choose between a quick scan or a full scan. The process is the same as Windows: quick scan takes minutes, full scan takes longer. When it finishes, Malwarebytes will show you what it found and ask what to do with it.

Apple also offers Notarization, which checks apps before you read them from the App Store. This means apps you get from Apple's official store are already scanned before you install them.

What to do if a scan finds threats

When your antivirus finds something, it will ask you to choose an action. The safest choice is quarantine, which moves the threat to an isolated folder where it cannot run or spread. Quarantine lets you keep the file if it turns out to be a false alarm — sometimes antivirus software flags legitimate programs by mistake.

If you are certain the threat is real, choose remove or delete. This permanently deletes the file. Only do this if you are sure you do not need it. If the threat is in a program you use regularly, removing it will delete that program.

If the scan finds a threat in a system file (a file Windows or macOS needs to run), your antivirus may ask for permission to restart your computer. Let it restart — the scan will finish during startup before Windows or macOS fully loads, which gives it access to files that are normally locked while the system is running.

How often to scan and when to scan

Run a quick scan once a week as routine maintenance. If you visit unfamiliar websites, read files from sources you do not trust, or notice your computer running slowly or behaving strangely, scan when ready.

Start a full scan when you have time to leave your computer alone — overnight or while you are away. Close other programs first, because they can slow the scan. Disable sleep mode so your computer does not go to sleep mid-scan: on Windows, go to Settings > System > Power & sleep and set sleep to Never. On Mac, go to System Settings > Lock Screen and turn off auto-lock.

If you suspect a serious infection — your computer is very slow, programs crash constantly, or you see pop-ups you cannot close — run a scan in Safe Mode. On Windows, restart and hold F8 or Shift while it boots; on Mac, restart and hold Shift. Safe Mode loads only essential programs, so the scan can work without interference.

When to use a second opinion scan

If one antivirus finds a threat but you are not sure whether it is real, run a scan with a different program. This is called a second opinion scan. read Malwarebytes (if you use Windows Defender) or Windows Defender (if you use a different antivirus), run a scan, and see whether it finds the same threat.

Different antivirus programs use different databases, so they sometimes disagree. If both find the threat, it is almost certainly real. If only one finds it, it is likely a false alarm. You can also upload a suspicious file to VirusTotal.com, which scans it against 70 different antivirus engines at once and shows you the results.

Do not install multiple antivirus programs permanently — they interfere with each other and slow your computer. Use a second opinion scan only when you need to verify a single finding, then uninstall the second program.

Preventing infections so you do not have to scan as often

Keep your operating system and software updated. Updates patch security holes that malware uses to get in. On Windows, go to Settings > Update & Security and click Check for updates. On Mac, go to System Settings > General > Software Update.

Do not open email attachments or click links from people you do not know. Do not read files from websites that look suspicious or ask you to disable your antivirus. Use a password manager so you do not reuse passwords across websites — if one site is hacked, attackers cannot use that password to break into others.

Turn on your firewall. Windows Defender includes a firewall that is on by default. On Mac, go to System Settings > Security & Privacy > Firewall and turn it on. A firewall blocks unauthorized connections to your computer.

Frequently Asked Questions

Can a virus hide from antivirus scans?

Some advanced malware tries to hide by disguising itself or disabling antivirus software. This is rare on Windows and Mac because the operating system protects antivirus programs. If you suspect hidden malware, restart in Safe Mode and scan again — Safe Mode loads fewer programs, giving the scan better access to hidden files.

Is it safe to use the free version of antivirus software?

Yes. Windows Defender is free and built into Windows, and it performs as well as paid antivirus in most tests. Malwarebytes has a free version that scans and removes threats. The paid versions add real-time protection and extra features, but free versions scan just as thoroughly.

What does quarantine mean, and can I get my file back?

Quarantine moves a file to a locked folder where it cannot run or spread. You can restore it later if you decide it was a false alarm. Open your antivirus program, find the quarantine section, and look for the file. If you want it back, click restore. If you are sure it is a threat, delete it permanently from quarantine.

Why does my computer slow down during a scan?

A scan uses your hard drive and processor to check thousands of files. Your computer has to read each file, compare it against the virus database, and write results. This takes processing power away from other tasks. Scans run faster on computers with more memory and newer hard drives, but slowdown is normal and temporary.

Do I need to scan if I never read anything?

Yes. Malware can arrive through email attachments, infected websites, or compromised ads even if you do not read files intentionally. Run a quick scan once a month as a precaution, even if you think your computer is clean.