I was using AVG until I tired of it asking me to restart my system after every update. Now I use ClamWin, a lightweight open source virus scanner. The only downside to ClamWin, and it's a big one, is that it doesn't have real-time scanning. So if you have users downloading all sorts of shady stuff or popping in contaminated USB drives, then you should try something else.
Avast is also good.
But for the love of baby Jesus, do not let Norton touch one of your machines. It will slow down and wreck your system more than any virus or spyware out there.