The best anti virus overall is AVG 8.0 if you don't mind your computer being a bit slow...BUT
if you want your computer to run at the same or a little faster (once the viruses/adware) is gone you want avast! with a cool firewall called Zclient (google it) it is a firewall with an added anti virus program so two anti virus programs for the price of none?
I like it AND it has an added firewall that works very well in my opinion... AND it didnt slow down my comp at all!
now the worst anti virus is Norton and Mcafee (If they arent the same I get confused with the two) and the worst fire wall is not having one at all xD