A few of my favorite & most used free tools I use & recommend to my clients
Spybot Search & Destroy - Excellent, free program, one of the most popular & best spyware removal programs - and totally free.
HiJackThis - Excellent, one of the best tools for manual removal of various startup processes in a windows computer - free. Requires some technical skill - be carefull, you can cause other problems if you don't know what you're doing.
TheKillbox - Excellent tool to manually delete files in use. Some technical skills required. Be careful with this! It will let you remove most files, including ones you may need.
Lavasoft AdAware - Excellent program, free for personal use. There is also a more comprehensive paid version available here
CWshredder - Excellent at what it does. Removes most varieties of Cool Web Search - purchased by a commercial company recently, but the cwshredder part is still free
Crap Cleaner - Not for spyware removal, but a nice freeware program that removes cookies, temporary files, history files, etc.