Microsoft is currently offering their Windows AntiSpyware software for free, at least until July 31, 2005 for free. Microsoft acquired the technology from a company called GIANT Company Software, which they bought recently. This company made one of the better Anti-Spyware scanners on the market.
I personally use this software, and I am very impressed with it. It not only finds more spyware, adware, etc. It also has very extensive proactive features, to help prevent you from being infected in the first place.
This software is now part of the everyday arsenal to protect myself while on the Internet.