understanding viruses and malware

This is usually an extremely geeky subject, but this article from El Reg is more plain-english than most:

Zombie machines infected with Trojan horse malware can be used to relay spam or launch denial of service attacks. Compromised machines can be also be pointed to websites from which additional items of malware can be downloaded. The practice is normally used to update Trojan code, but it also creates a means for cybercrooks to make a "nice little earner". The income that can be earned grows with the numbers of installs, and varies based on the geographical location of an installation. For example, installing spyware on 1,000 machines in Australia earns $100 but only $50 in the US, and a measly $3 in Asia. A sample price list obtained by net security services firm sheds fresh light on the phenomenon.

The scope of botnets is growing. It's why i make Kev use Linux on the home system.