there are two that you should use.
1) adaware -
http://www.lavasoftusa.com/software/adaware/
get it from this site. there is another "adaware" program out there that may INSTALL spyware. that program is trying to trick people into installing it, instead of the "good one."
adaware is free, but you can buy the addon that constantly scans your computer for ad/spyware.
2) spybot -
http://www.safer-networking.org/en/index.html
also a good one. i have BOTH on my computer and run them once a week or so. they appear to scan for many of the same things, but there are somethings one gets that the other does not.
i have never had any problems with either of these programs.
last make sure you have an anti-virus program AND THAT YOU UPDATE THE DATA FILE REGULARLY.