Software: RUBotted (Anti-Botnet Scanner)

Trend Micro has release a free anti-botnet detection tool called RUBotted. This software can help prevent your computer from being taken over and used for malicious purposes such as sending spam or remotely attacking other computers.

Here is excerpt from Trend Micro site about their product:

"Malicious software called Bots can secretly take control of computers and make them participate in networks called “Botnets.” These networks can harness massive computing power and Internet bandwidth to relay spam, attack web servers, infect more computers, and perform other illicit activities."

If you have never heard of a Botnet, here is an article from Wikipedia that discusses it in more detail.