spybot search and destroy: http://www.majorgeeks.com/download2471.html
these are all free and great to use...just download them, install,update, and run
spywareblaster stops stuff from installing and runs in the background with very little resources.
The others need to be ran and updated at least once a week depending on usage.
This guy warns if something is trying to install to registry and also gives a good start-up program manager to control unneccesary programs from just starting on their own.
this guy is a registry cleaner and will remove old keys and file pathes that no longer exist or are any good. This program is safe but let it find and fix as it knows the bad issues.
this guy removes hidden url files and cookies and dat files and history files that allow spyware to learn about what you do and see on the computer.It will remove password files too so make sure you know what all your passwords are.