You can try Kaspersky 7.0 it's a very good Anti-Virus software also....(i'm using it!) I switched to this Anti-Virus because of good reviews at it was recommended to have it like NOD32 and Norton....
Well if you want free Anti-Virus try AVG Free 7.5, it's a good Anti-Virus software and I have it before(yet mine is licensed software) but for me I recommend Kaspersky 7.0 because it doesn't slow down your computer also, Automatic updates not just daily,Startup scan, and more...

Hope this helps
um can I ask what OS X version you are using btw....Leopard??Tiger???