How to Know if Your Computer is Infected with a Virus

Computer infection
Many times you can tell if your computer has a virus, but it's rarely this obvious.
© iStockphoto/YanC

We all know computer viruses -- and other kinds of malware -- can cause problems ranging from irritating to catastrophic. Some malware replicates itself until it fills up all available space on your hard drive, turning your computer into a brick. Other kinds corrupt data on your machine or make your computer unstable. A few will even attempt to use your e-mail programs to distribute the malicious code to everyone in your contacts list. And there's always the possibility a cracker -- that's a malicious hacker -- will use malware to get remote access to your computer.

No one wants to own a computer infected with a nasty virus. That's why it's very important to practice safe computing habits and to install reliable anti-virus software. You can avoid most malware just by paying attention and staying away from a few common traps. If your anti-virus software is up to date, you should be in pretty good shape.


But once in a while, computer viruses get beyond our defenses. Maybe our anti-virus software is out of date or has been compromised by a particularly clever bit of code. Perhaps we clicked on a link by accident and activated a virus. Or someone else used our computer and downloaded some malware by mistake.

How do you know if your computer has been hit by a computer virus? If your anti-virus software is robust and up to date, you'll likely receive a message as the application scans your computer. That makes detecting the virus a breeze. But what if your software is out of date or the virus has managed to deactivate the anti-virus program? Are there signs you can watch out for that might indicate a virus?

As a matter of fact, there are several signs that could indicate the presence of malware on your computer. We'll take a closer look on the next page.


Signs of a Computer Virus

Computer infection
Computer viruses range from merely irritating to outright dangerous.
© iStockphoto/borisyankov

Assuming your anti-virus software hasn't alerted you to the presence of a virus, here are some indicators of malware on your computer:

If your computer has become unstable, that's a sign that something's wrong. Some malware messes with important files that keep your computer running properly. That could cause your computer to crash. If your computer crashes when you try to run a specific application or open a particular file, that tells you that something has corrupted the data. It could be malware.


Does your computer seem to run much more slowly than it used to? This could be the result of malware as the malicious code begins to drain your computer's processing resources. If you aren't running a resource-heavy application but your computer is very slow, you might have a computer virus.

Strange messages indicating that you can't access certain drives on your computer are another sign that something is wrong. In a similar vein, applications that won't run or files that won't open may also be the result of infection. Other indicators include hardware (like printers) that no longer respond to commands. While none of these guarantee the presence of a virus, they do suggest that something is wrong with your machine.

If you notice that file sizes are fluctuating even if you aren't accessing those files, that's another sign of a computer virus. And finally, if you access menus and their appearance is odd or distorted, you could be the victim of a malware attack.

It's important to remember that computer viruses are one potential cause of problems like the ones we've listed here, but that they aren't the only cause. If you believe your computer has been infected by a virus, don't panic. Follow the steps we suggest in How To Remove a Computer Virus. You might lose some data in the process but you shouldn't lose everything.

Learn more about computer viruses and other topics on the next page.


Lots More Information

Related HowStuffWorks Articles

  • Dittrich, David. "Lifecycle: Preventing, detecting and removing bots." March 20, 2005. (March 19, 2009),289483,sid14_gci1068906,00.html
  • Microsoft. "Signs of viruses: Are you infected?" Sept. 20, 2006. (April 20, 2009)
  • Quarantiello, Laura. "Computer Virus Warning Signs." Internet World Stats. (April 20, 2009)
  • Robertson, Jordan. "How to tell, what to do if computer is infected." AP News. March 15, 2009. (March 17, 2009)