SysInternals may be known to Ask Jack readers as I use and keep recommending Process Explorer. However, the company has become much more famous recently for starting the firestorm of criticism aimed at Sony BMG's use of a "rootkit" technique to hide the copy-protection system on some of the audio CDs released in the US. (The EFF has a list of the albums affected.)
SysInternals explains Rootkit Revealer in this Introduction, and you can download it using a link at the bottom of the page.
I ran it this weekend just for the experience, and found I don't have any rootkits installed. Just as well, really, as the Rootkit Revealer doesn't seem to have a way of fixing them, and I wouldn't know how to do it manually....
Both Process Explorer and Rootkit Revealer are freeware.
