Free Anti-Virus Comparison Review @ OCC
 

As the other feature-rich antivirus, Avast! Antivirus comes in third, performance wise, to AVG. Personally though, my preference would be to consider Microsoft Security Essentials as third, but that may be my own bias talking. Avast definitely has a rich feature set which may go very far in protecting you, but I just found it too intrusive for my taste. Of course MSE is lacking in both features (not always a bad thing) and performance, so really only its simplicity and the Microsoft name gives a recommendation for it much weight. Perhaps it would be best to say Avast has won at this moment in time, but if MSE can get its detection-act together then the table may turn.

http://www.overclockersclub.com/reviews/av_comparison/