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.