Most of these people Nathan, are just sad sorry individuals with nothing better to do than try to ruin others PC's
A few years ago, this was limited to a small number of "spotty" individuals hiding away in their bedrooms, but it is more widespread now. Very annoying really
A simple example of a virus is a small BASIC (Beginners All-purpose Symbolic Instruction Code) program that instructs the screen to fill with 'a particular word/phrase', easy to do, but quite annoying for the many shop owners that suffered this problem in the 1980's
That type of thing couldn't do any damage though, just a looping effect on the computer it was written on