Spy Bot and Adware are really good ones and they are both free. snapfiles.com and freeware files.com are sites I use for all my free tools (majorgeeks too).
Another good (free) tool to have is Ccleaner, which cleans all the garbage off your pc like temp files, log files, MRUs, etc.
Also, there are some good (free) firewalls to be had as well and they will block stuff from getting in too.
You won't have to look to hard to find good free tools that will do all kinds of stuff for you.