AVAST! is a great free antivirus as well. It auto updates a few times a day. I have used this free program for over 5 years and has saved my butt a few times. Very easy to use.
I have used AVG for years until it quit downloading all the newest definitions. Because of that, I ended up with a virus from HELL. I have talked to several people and they all had the same prob so they switched over.
It has been over a year now and I am using Avast and IMO, it is the best of the best.