Antivirus Says “No Threats Found,” but Your PC Still Acts Strange? Here's What That Means
You ran the scan, it came back clean, and the computer is still doing things you didn't ask it to. That happens a lot, and it doesn't mean your antivirus is broken. It means a scan answers one question, and your computer is asking a different one.
What a Clean Scan Actually Tells You
It's a useful answer to a narrow question.
Antivirus software compares the files and programs on your computer against a list of things it already knows are bad, plus some behavior rules for catching new ones. “No threats found” means nothing on the machine matched that list at the moment of the scan. That's worth knowing, and Microsoft's own guidance is to keep Windows Security turned on and run those scans.
What a clean scan does not tell you: whether a browser extension is quietly changing your search results, whether a website has permission to push notifications that look like virus alerts, whether a legitimate program is misbehaving, or whether a failing hard drive is why everything takes forever. None of those are malware in the sense your antivirus is looking for, so it doesn't flag them. Some real infections also hide from a scan run inside a copy of Windows they've already compromised, which is why I scan machines offline at the bench.
What Else Makes a PC Act Strange After a Clean Scan
Here's what I find most often when someone brings in a computer that “passed” its scan.
| What you're seeing | Common cause that a scan won't flag | Malware possible? |
|---|---|---|
| Pop-ups in the corner of the screen, even with the browser closed | A website you clicked “Allow” on is sending browser notifications. Chrome, Edge and Firefox all let you revoke that per site. | Sometimes; scam pages use this trick |
| Search results go to a search engine you didn't pick | An unwanted browser extension or a changed default search setting. Often bundled into a free download. | Yes, this is the browser hijacker category |
| Programs open slowly, everything lags | A nearly full drive, a failing hard drive, too little RAM, or dozens of startup programs. | Occasionally |
| Random crashes and freezes | Bad driver update, overheating, failing memory or storage. | Rarely |
| Fan runs hard while you're doing nothing | Windows Update or indexing in the background, or a cryptominer that a scan missed. | Yes, worth checking |
| “Your computer is infected, call this number” full-screen page | A scam web page, not an infection. It has no phone number to call and nothing to install. | Only if you downloaded what it offered |
Any one of these on its own can be innocent. Two or more together, or one that keeps returning after you fix it, is when I'd stop guessing and have it looked at.
Three Things Worth Trying First
Ten minutes, no downloads, nothing that can make it worse.
Look at your browser's extensions and notification permissions
In Chrome or Edge, open the menu, go to Extensions, and remove anything you don't recognize or didn't install on purpose. Then check Settings, Privacy and security, Site settings, Notifications, and remove any site you don't want alerts from. Google's help page on unwanted ads and pop-ups walks through the same steps.
Run a second opinion scan from Microsoft
Microsoft Safety Scanner is a free, standalone tool from Microsoft that runs on demand and doesn't replace your antivirus. Download it from Microsoft's own site, never from a link in a pop-up.
Check how full the drive is
Open File Explorer and click This PC. If the C: drive bar is red or you have less than about 10 percent free, that alone will make a computer crawl. Microsoft's free-up-space page covers safe cleanup.
What an In-Office Assessment Sorts Out
Performance problem, security problem, or both. I check for all of it in one visit.
When a computer comes to my bench in Somerville, I don't just rerun the scan you already ran. I scan the drive from outside Windows so nothing on the machine can hide from it, I go through the startup entries, scheduled tasks and browser profiles by hand, and I test the drive's health and memory. That's how you tell a hijacked browser from a dying hard drive from an actual infection, because all three can look the same from the keyboard.
The diagnostic is $75 and it's credited toward the repair. If it turns out to be malware, cleanup is a flat $149 with the diagnostic folded in, and most machines are back in 24 to 48 hours. If it's a full drive or a failing one, you'll hear that instead, with a price, before anything happens. The full breakdown is on my virus removal page, and if it's more of a speed problem than a security one, how to fix a slow computer goes deeper.
Bring the computer, the charger, and a note of what the symptoms are and when they started. If the strange behavior is tied to a particular website or program, tell me that too. Dave's Computers is at 75 N Bridge St in downtown Somerville, behind Bank of America. Walk in Mon-Fri 10-5 or Sat 9-2, no appointment needed, and bring the charger.
Directions to the Shop in Somerville
Drop-off only, no appointment needed. Here's how far you are and how to get here.
From Bridgewater
Route 22 or Route 206 south into downtown Somerville, then N Bridge St.
Open in Google MapsFrom Hillsborough
Route 206 north into Somerville; N Bridge St is one block off Main Street.
Open in Google MapsFrom Bound Brook
Route 28 west straight into Somerville.
Open in Google MapsFrom Piscataway
Route 28 west, or I-287 north to Route 22 west, into downtown Somerville.
Open in Google MapsFrom Flemington
Route 202 north into Somerville; downtown is just past the Somerville Circle.
Open in Google MapsFrom Princeton
Route 206 north the whole way into Somerville.
Open in Google MapsServing Somerset, Hunterdon, Middlesex and Mercer counties from one shop. Bring the computer and its charger, walk in during business hours, and I'll take it from there. Call (908) 428-9558 first if you need it back the same day.
Clean Scan, Strange PC: Common Questions
If my antivirus says no threats found, can I still have a virus?
Yes, though it's less likely than the symptoms make it feel. Some malware is built to hide from a scan run inside Windows, and plenty of unwanted software (hijacker extensions, notification spam) isn't classified as a virus at all. A clean scan is a good sign, not a guarantee.
Should I install a second antivirus program?
No. Two real-time antivirus programs fight each other and slow the machine down. A one-time second-opinion scanner such as Microsoft Safety Scanner is fine because it doesn't run in the background. Download it only from Microsoft's site.
My browser keeps showing virus warnings but the scan is clean. Which is right?
Almost certainly the scan. Warnings that appear inside a browser tab, especially ones with a phone number or a countdown, are web pages, not alerts from your computer. Close the browser. If they come back on their own, a notification permission or extension is behind it.
How much does it cost to have Dave's figure out what's wrong?
The diagnostic is $75, credited toward whatever repair you approve. Virus and malware removal is a flat $149. Everything is quoted before work starts, and the details are on the virus removal page.
Do I need an appointment to bring my computer in?
No. Dave's Computers is a drop-off shop at 75 N Bridge St in Somerville, NJ, behind Bank of America. Walk in Mon-Fri 10-5 or Sat 9-2 and bring the charger. Call (908) 428-9558 if you need it back the same day.
Sources
- Microsoft Support, Stay protected with Windows Security
- Microsoft Learn, Microsoft Safety Scanner
- Google Chrome Help, Remove unwanted ads, pop-ups and malware
- Google Chrome Help, Use notifications to get alerts (how to turn site notifications off)
- Microsoft Support, Free up drive space in Windows
- Federal Trade Commission, How To Recognize, Remove, and Avoid Malware
Scan Says Clean, Computer Says Otherwise?
Bring it to Somerville. $75 diagnostic credited toward the repair, $149 flat for malware cleanup, most machines back in 24 to 48 hours.
📞 (908) 428-9558