Norton and McAfee are terrible, they are bloated hogs that squeeze all of the life out of your computer.
AVG is fine, but is full of holes.
If you want the best, then you need to pay for it. The option that Starrman mentioned in this thread is the best antivirus software that I've ever run on a network, namely Eset NOD32.
It's very light on resources, it updates itself quietly in the background, and thanks to some very smart tech it picks up sparkling new viruses that nobody even knows about.
http://www.eset.com/
(Whatever you do, stay the HELL away from Trend Micro!)
