spybot is probably the best free spyware program out.

Another good tool is the security task manager from neuber:
http://www.neuber.com/taskmanager/

It is free but you have to put up with some "trial messages" unless you want to pay 29 bucks to get rid of them.

Edit: Oops, looks like it has been changed. The "free" version is just a trial version and you must purchase the actually software for full functionality

-David