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Symptoms of Virus Infection
There is usually no simple way to know when you have a virus. I should rephrase this: some viruses exhibit behavior that tell you immediately that they are on your system, but so many viruses mimic other system problems that it is most accurate to say "there is usually no way to rule out a virus as a possible cause of strange software or system behavior on your PC".
Virus writers are usually pranksters whose software is normally designed to avoid detection, and to cause mischief. Both of these mean that viruses are designed to mimic natural processes in your PC so that you don't think you have an infection. They often make your system behave in strange ways, but in strange ways that could also be the result of an application bug, driver glitch, or even a hardware problem. For example, a virus can make strange things print on the screen--but so can a driver problem, or a bad BIOS setting. A virus can make the system spontaneously reboot--but so can a bad power supply, or an operating system problem, or an overheating processor.
Since viruses are so common, and they can exhibit so many strange types of behavior, and so many of these behaviors are similar to other, real hardware and software problems, I always recommend a virus scan as the first step in troubleshooting a hardware or software problem. Until virus infection has been cleared as a potential source of problems, it doesn't make much sense to look for a real hardware or software cause. For example, many symptoms of resource conflicts are similar to the symptoms of virus infection. The following are the sorts of problems that are usually real system or component problems, but can also be a result of virus trickery (this list is not exhaustive; virus writers are very creative):
There are some types of strange system behavior that hint much more strongly that a virus is around and responsible for the problem. Seeing any of the following types of behavior on your system should send you scrambling for your clean antivirus boot disks, as they are not normally caused by legitimate hardware or software problems: