Run a scan with Windows Defender or your antivirus software

The fastest way to check for viruses is to use the antivirus program already on your computer. Windows Defender comes built into Windows 10 and Windows 11 at no cost. If you use a Mac, the built-in Gatekeeper and XProtect handle basic scanning. If you have paid antivirus software installed — Norton, McAfee, Kaspersky, or another brand — open that program instead.

In Windows, open Settings, go to Privacy & Security, then select Windows Security. Click Virus & threat protection. Under Current threats, select Scan options, then choose Quick scan to check your most-used files and folders. A quick scan usually takes 5 to 15 minutes. If you want a deeper check, select Full scan instead — this examines your entire hard drive and can take an hour or more.

On a Mac, open System Settings, go to General, then select Software Update. Your system will check for security updates. You can also open Activity Monitor to see what programs are running and whether any look unfamiliar or are using unusual amounts of memory or processing power.

Key Takeaways

  • Windows Defender and Mac's built-in protection can scan your computer for viruses without paying for additional software.
  • A quick scan checks your most-used files and takes 5 to 15 minutes; a full scan examines your entire drive and takes longer but is more thorough.
  • If you see unfamiliar programs running in Activity Monitor or Task Manager, note their names and search for them online to determine whether they are legitimate.
  • Restart your computer in Safe Mode before scanning if you suspect a serious infection, because some viruses do not run in Safe Mode.
  • If a scan finds threats, your antivirus software will offer to remove or quarantine them — follow the on-screen instructions to complete the removal.

Restart in Safe Mode for a deeper scan

If your computer is running slowly, crashing often, or showing pop-ups you did not install, restart it in Safe Mode before scanning. Safe Mode loads only the essential programs Windows needs to run, which prevents most viruses from starting up. This gives your antivirus software a clearer view of what is actually on your drive.

In Windows 10 or 11, hold the Shift key and click the power button in the Start menu, then select Restart. When the screen shows troubleshooting options, select Troubleshoot, then Advanced options, then Startup Settings. Click Restart. When your computer restarts, press 4 or F4 to enter Safe Mode. Once you are in Safe Mode, open your antivirus program and run a full scan.

On a Mac, restart and when ready hold Command + S. This boots into Single-User Mode, which is similar to Safe Mode. Type fsck -fy to check your hard drive, then type reboot to restart normally. However, most Mac users do not need to do this — the built-in protections are usually sufficient.

Check what programs are running and remove unfamiliar ones

Viruses and malware often hide as background programs. Open Task Manager in Windows by pressing Ctrl + Shift + Esc. Look at the Processes tab and note any programs you do not recognize. Search the program name online — if multiple sources say it is malware or spyware, you should remove it.

To uninstall a program in Windows, go to Settings, select Apps, then Apps & features. Find the program in the list, click it, and select Uninstall. Follow the on-screen steps. If the program will not uninstall or keeps reappearing, your antivirus software may need to remove it first — run a full scan and let the program quarantine or delete the threat.

On a Mac, open Activity Monitor by pressing Command + Space, typing Activity Monitor, and pressing Enter. Look at the CPU and Memory tabs to see what is using the most resources. Unfamiliar programs using high CPU or memory may be malicious. To remove a program on a Mac, open Finder, go to Applications, find the program, and drag it to the Trash.

Update Windows, macOS, and your browser

Viruses often enter through security holes in outdated software. Keeping your operating system and browser current closes many of these holes. In Windows, go to Settings, select Update & Security, then Windows Update. Click Check for updates and install any available patches. Your computer may need to restart.

On a Mac, open System Settings, go to General, and select Software Update. Install any updates shown. For your browser — whether Chrome, Firefox, Safari, or Edge — check the menu for an About or Help section. Most browsers check for updates automatically and will prompt you to restart the browser when updates are ready.

Viruses also spread through outdated plugins like Flash or Java. If you have Java installed and do not use it, uninstall it. For other plugins, visit the publisher's website and read the latest version.

Use a second opinion scanner if you are still concerned

If Windows Defender or your antivirus software finds nothing but your computer still behaves oddly, run a second scan with a different program. Malwarebytes is a common choice and offers a free version. read it from the official Malwarebytes website, install it, and run a scan. Different antivirus programs catch different threats, so a second opinion can reveal what the first scan missed.

Do not install multiple antivirus programs to run at the same time — they will conflict with each other and slow your computer. Instead, uninstall your current antivirus, restart, install the new one, and run a scan. Then decide whether to keep the new program or reinstall your original one.

Prevent future infections

After you have scanned and removed threats, take steps to avoid reinfection. Do not open email attachments from people you do not know. Do not read files from untrusted websites. Be cautious of links in emails, texts, and social media — hover over a link to see where it actually goes before clicking. If a website looks suspicious or your browser warns you about it, leave when ready.

Enable automatic updates in Windows and macOS so security patches install without you having to remember. In Windows, go to Settings, Update & Security, and make sure automatic updates are turned on. On a Mac, go to System Settings, General, Software Update, and check Automatic Updates.

Use a strong, unique password for each online account. If a website is breached and your password is stolen, a unique password means the attacker cannot use it to access your other accounts. Consider using a password manager like Bitwarden or 1Password to generate and store strong passwords.

What to do if your computer will not start or scan

If your computer will not start normally or your antivirus software will not run, you may have a severe infection. Try restarting in Safe Mode and running a scan from there. If that does not work, you can create a bootable antivirus disk on another computer and use it to scan your infected machine. Kaspersky, Bitdefender, and other antivirus makers offer free bootable rescue disks you can read and burn to a USB drive.

If you are not comfortable doing this yourself, take your computer to a local repair shop. They have tools and experience to remove stubborn infections. The cost is usually between $50 and $150 depending on how severe the infection is.

Frequently Asked Questions

How often should I scan my computer for viruses?

If you have antivirus software running and automatic updates turned on, a monthly full scan is usually enough. If you read files often or visit risky websites, scan weekly. If your computer shows signs of infection — slowness, pop-ups, crashes — scan when ready.

Will scanning for viruses slow down my computer?

Yes, but only while the scan is running. A quick scan takes 5 to 15 minutes and uses some processing power. A full scan can take an hour or more. Once the scan finishes, your computer returns to normal speed. If your computer is slow even when no scan is running, the slowness is likely caused by the virus itself, not the antivirus software.

What does quarantine mean?

Quarantine means your antivirus software has isolated a file so it cannot run or spread. The file is still on your computer but locked away. You can view quarantined files in your antivirus program and delete them permanently, or restore them if they were flagged by mistake. Most of the time, you should delete quarantined files.

Can I scan my computer while using it?

Yes, but your computer will run slower during the scan. If you are working on something important, wait until you are done or run the scan overnight. You can also schedule scans to run at a specific time — most antivirus programs have a scheduling option in their settings.

Is Windows Defender good enough, or do I need paid antivirus software?

Windows Defender is sufficient for most users who practice safe browsing habits. Paid antivirus software often includes extra features like a VPN, password manager, or identity theft protection, but these are not necessary for basic virus protection. If you read files often or visit unfamiliar websites regularly, paid software may offer more thorough scanning, but Windows Defender catches the majority of common threats.