According to Wikipedia, a virus is: "...a self-replicating computer program that spreads by inserting copies of itself into other executable code or documents... In common parlance, the term virus is often extended to refer to worms, trojan horses and other sorts of malware... While viruses can be intentionally destructive, for example, by destroying data, many other viruses are fairly benign or merely annoying. The predominant negative effect of viruses is their uncontrolled self-reproduction, which wastes or overwhelms computer resources."
If you suspect that you may have a virus on your computer you can download any of the following utilities used to remove it.
Here is a listing of the most recent viruses and virus alerts on the internet:
Top 10 Viruses