A virus is a small program that once on your machine causes it to act in ways that are undesirable. They are common in many downloaded files and they are called virus because they try to replicate themselves. Damn annoying. Some come in though email too and they can prevent hardware working properly, even use your computer to send email!
This is why an anti virus program is important. I recommend Grisofts AVG. A free version is available (avgfree) for home users (free.grisoft.com) and it is excellent.
Mart9012
June 2007