UPDATE:  Adaware is no longer recommended by CS.  Bloat and lack of decttion has added to this programs demise.  Try malware bytes for your free spyware dectection needs.

Lavasoft's Ad-aware program has been in the malware game since the beginning.  The basic still remains to be free and while it does not scan your background files it does do a fine job when you remember to run it.  The update does run in the background and continue to keep you up to date with the latest malware detections. 

I have been using this software for years and continue to recommend it to everyone. Its very easy to use and its free. The ones you pay for provide more advanced functions but you don't really need them. Check it out here: