A common way for cyber-criminals to make money on the net is by renting out something called a bot-net which can be used to deliver spam, serve up web pages or take down rival sites.

A bot-net is basically a distributed group of 1000's of PCs all running some kind of virus software designed to make the infected computer come under the control of the bot-net owner.

This can be bad news for the owner of the infected PC because attacks on sites appear to originate from that person's computer and in worst case scenarios an infected computer might be used to serve up illegal webpages such as child pornography or terrorism related material.

The viruses that run this software on your computer are often introduced by spam emails.