And you thought spammers and spyware was not bad enough, according to the story http://theregister.co.uk/content/55/35722.html:

German magazine c't says it has evidence that virus writers are selling the IP addresses of PCs infected with Trojans to spammers. Spammers use these infected systems to unlawfully distribute commercial email messages...
Okay, that's intersting... and it gets better:

The Trojan involved was spread by a virus called Randex. This small program contacted its 'master' through the chat protocol IRC. It was programmed to look for CD keys of games, or secretly load additional software. The Trojan was also able to install a proxy server which can be used to relay spam through the infected PCs.

A college student managed to track down the distributor of a computer virus in the UK, and the editorial staff of c't was then able to buy access to the infected machines.
Which goes to show the importance of updates, patches, firewalls, AV, and not running anything that you do not know is safe. While this will not catch everything, it does help a bit. Also, this is a pretty big allegation, but I am surprised this has not been used earlier because an "infected" machine is in most cases a server the user (or admin) does not know exists, and if it is a server ready to serve it is an open book for all kinds of nasties including more spam and ads... ironically, it is most likely an ad like "LEARN HOW TO REMOVE THESE ADS BY CLICKING HERE!!"

Information on Randex: http://www.f-secure.com/v-descs/randex.shtml