If you want the freebies in order of preference :- avast!, AVG, Antivir then Bitdefender.
avast! best updates, best e-mail scan, good pass rate at virus bulletin, best freebie.
AVG good product, new version 7 better than its predecessor good reliable product, I just rate avast as better.
Antivir, never passed virus bulletin tests, updates frequently involve replacing the entire program and e-mail coverage not as good as avast or AVG, better than nothing though.
Bitdefender, the full version is OK, but they hacked off too many features to come up with the free version, best avoided.