What scanning your PC actually does

A virus scan searches your hard drive, memory, and running programs for code that matches known malware signatures — patterns that antivirus companies have already identified as harmful. When you run a scan, the software compares files on your computer against a database of known threats. If it finds a match, it flags the file and gives you options to remove it, quarantine it, or ignore it.

Most modern PCs come with built-in scanning tools. Windows includes Windows Defender (called Microsoft Defender on Windows 10 and later), and macOS includes XProtect. These work in the background automatically, but you can also run a manual scan whenever you want. The scan takes anywhere from a few minutes to an hour depending on how much data you have and which type of scan you choose.

A scan does not may provide your PC is completely clean — new malware appears constantly, and some sophisticated threats hide from standard scans. But regular scanning catches the vast majority of common infections and gives you visibility into what is actually on your machine.

Key Takeaways

  • Windows Defender (built into Windows 10 and later) runs automatic scans in the background and can also be opened manually for a full system scan.
  • A quick scan takes 5 to 15 minutes and checks only the most common infection points; a full scan can take an hour but examines every file.
  • If a scan finds something, read what the software recommends before clicking remove — some flagged items may be legitimate programs you use.
  • Running a scan monthly or after visiting unfamiliar websites gives you regular visibility into what is on your PC without slowing it down significantly.

How to run a scan on Windows

Open Windows Defender by typing "Windows Defender" or "Microsoft Defender" into the search box at the bottom left of your screen, then click the app when it appears. Once it opens, look for the shield icon on the left side and click "Virus & threat protection."

You will see two scan options. "Quick scan" checks the folders and memory locations where malware most commonly hides — this takes about 10 minutes and is fine for a routine check. "Full scan" examines every file on your drive — this can take 30 minutes to an hour but is more thorough. Click whichever you prefer, then click "Scan now." Your PC will continue working while the scan runs, though it may slow down slightly.

When the scan finishes, Windows Defender shows you a summary. If it found nothing, you will see "No threats found." If it found something, click on the result to see details. The software usually recommends "Remove" for anything it flags — follow that recommendation unless you recognize the file as something you intentionally installed.

How to run a scan on Mac

macOS does not have a built-in scanning tool you open manually the way Windows does. Instead, XProtect runs automatically in the background and scans files as you read them. You will not see a scan window or progress bar — the protection is silent.

If you want a more thorough check, you can read a free third-party scanner like Malwarebytes for Mac. Open the app, click "Scan," and let it run. This takes 10 to 20 minutes and checks your entire system. If it finds anything, follow the prompts to remove it.

Most Mac users do not need to manually scan because XProtect is effective at catching threats before they install. But if you have downloaded files from unfamiliar sources or suspect something is wrong with your Mac, running Malwarebytes gives you peace of mind.

Understanding what the scan results mean

When a scan completes, you get one of three outcomes: "No threats found," a list of flagged files, or a notification that something was already quarantined. "No threats found" means the scan did not match any files against its database of known malware — this is the result you want.

If the scan flags something, the software shows you the file name, location, and threat category. Common categories include "Trojan," "Worm," "PUP" (Potentially Unwanted Program), or "Adware." Read the threat name carefully — if it is something you recognize and intentionally installed, you can choose to ignore it. If you do not recognize it, click "Remove" or "Quarantine." Quarantine moves the file to an isolated folder where it cannot run, which is safer than deleting it when ready in case you need it later.

If a scan says something was already quarantined, that means Windows Defender or another security tool caught and isolated the threat before you ran the manual scan. You can view quarantined items in Windows Defender under "Virus & threat protection" > "Manage settings" > "Exclusions," though most people never need to look at them.

When to scan and how often

Run a quick scan monthly or after you have visited websites you are unsure about, downloaded files from unfamiliar sources, or noticed your PC acting strangely — running slowly, showing unexpected pop-ups, or opening programs you did not click. A monthly routine scan takes 10 minutes and gives you regular visibility without much inconvenience.

If you suspect an active infection — your PC is very slow, you see constant ads, or programs keep crashing — run a full scan. Close all other programs first so the scan can focus its resources. Full scans are more thorough but take longer, so run one when you have time to let your PC work uninterrupted.

You do not need to scan every day. Windows Defender already runs automatic scans in the background on a schedule, usually at 2 a.m. if your PC is on. Manual scans are a backup check, not a replacement for that automatic protection.

What to do if a scan finds nothing but your PC still seems infected

If you ran a full scan and found no threats, but your PC still feels slow or behaves strangely, the problem may not be malware. Slowness often comes from too many programs running at startup, a full hard drive, or an aging PC that straightforward needs more resources. Pop-ups can come from browser extensions rather than system-level malware.

Check your startup programs by opening Task Manager (right-click the taskbar and select "Task Manager"), clicking the "Startup" tab, and disabling programs you do not need to run when your PC starts. Restart your PC and see if performance improves. If you use Chrome, Firefox, or Edge, check your installed extensions — malicious ones sometimes hide there even when antivirus scans find nothing.

If you remain concerned after checking startup programs and extensions, consider running a second opinion scan with a different tool. Malwarebytes (free version) or Kaspersky Rescue Disk are both reputable and sometimes catch things the first scan missed. But in most cases, a clean scan result means your PC is actually clean.

Frequently Asked Questions

Does running a virus scan slow down my PC?

A scan uses some processing power while it runs, so your PC may feel slightly slower during the scan. Once the scan finishes, performance returns to normal. Running a scan at night or when you are not using your PC avoids any inconvenience.

What is the difference between quarantine and remove?

Quarantine moves a file to an isolated folder where it cannot run or cause harm, but the file still exists on your PC. Remove deletes the file entirely. Quarantine is safer if you are unsure whether the file is actually malicious, because you can restore it later if needed.

Can I scan my PC while using it?

Yes, you can use your PC normally while a scan runs in the background. The scan will take longer and your PC may slow down slightly, but you can still open programs and browse. For a faster scan, close other programs and let the scan run uninterrupted.

Why does Windows Defender sometimes say "threats found" after I already scanned?

New malware can arrive between scans, or some threats hide from quick scans and only show up during a full scan. If you ran a quick scan before, try running a full scan. If the full scan also finds something, follow the removal prompts.

Is Windows Defender enough, or do I need a third-party antivirus?

Windows Defender is effective for most users and catches the majority of common threats. A third-party tool like Malwarebytes can provide additional scanning if you want extra assurance, but most people do not need both running at the same time — they can conflict with each other.