I think the best anti-virus software is the one sold by ESET. Unfortunately it not free, but the price for the complete suite which include firewall and the anti-virus with 3 licenses, the last time i paid was R$ 120,00 ( Brazilian currency ). I consider it a fair price. Their firewall solution i don´t think it is the best one, but the anti-virus, at least in my opinion is the best one. A long time ago, a friend of mine, was having some problem in his lab, and he tested many free anti virus softwares and they didn´t work out. When i came with this anti-virus, at that time i think it was called NOD32, he was able to clean all the virus that were spread in his lab network.