Is Your PC Part of a Botnet? - ReadWriteWeb
http://www.readwriteweb.com/archives/is_your_pc_part_of_a_botnet.php

Being part of a botnet is no fun. Your computer becomes your worst enemy, watching everything you do, collecting all of your secrets, and then delivering all that data to the bot-herder; the person who originated the network. But what does it really mean to be part of a botnet, and is there anything that can you do about it?

good information about determining if your pc is infected with malware