I think I'll add that microsoft recommends 2:
• Spybot-Search & Destroy (Spybot)
• Ad-aware (Lavasoft)
http://www.microsoft.com/mscorp/twc/privacy/spyware.mspx
We generally add 2 more ,both pay versions
Spysweeper
Pest patrol ( issue with false positives though)
It 's a gold mine out there for companies claiming to solve all your problems, Just give them money.
Some have been in business all of a few days.
Others like to spam forums.
Be careful out there.
How long have they been in business?
Do they use deceptive advertising?
Do they use look-a-like names hoping to cash in on ad-aware's reputation?
Or your error?
Adware Agent
AdwareHunter
AdwareSpy
NoAdware