I get a lot of email from people who believe their computer is infected by a virus. In most cases, it's not infected at all--evil software designers are still outnumbered by incompetent ones.
And even if there is malware involved, it's almost certainly not a virus.
The word virus refers to a very specific way that malware spreads from one PC to another. A computer virus infects an executable file, like a program, the way a biological virus infects a cell. When it gets the chance, it infects another file, and thus spreads.
Or perhaps I should say used to spread. Over the last few years, rogue programmers have found better ways to infect your computer, more suited for the Internet and email age. For instance, Trojans--programs that trick you into opening them, and infect your computer when you do--are quite popular among the tech-savvy criminal set.
Yet the word virus stays around. Why?
Because viruses were the most prominent form of malware when large numbers of people finally figured out that this was something to worry about. Everyone was talking about viruses in the 1990's. One of them destroyed an evil corporation in seconds, and another saved the world from alien invasion. (And no, I'm not going to tell you what movies I'm talking about; that would be spoiling.)
Thus, to the uninformed, the word virus came to mean any malicious computer program. It's like using the name Frankenstein to refer to the monster rather than the monster maker.
So check yourself before you tell someone your computer has a virus. You're probably admitting your own ignorance.
Or better yet, scan your PC to find and identify the malware. For a quick, free scan, I recommend the online Ewido scanner (which works only with Internet Explorer) or the free stand-alone program. I also like Kaspersky Lab's Web scanner. Another excellent choice, loved by many on the PCW Answer Line forum, is SUPERAntiSpyware, although unlike the others I mentioned, this one has to be installed.