I have Norton Internet Suite. It cost $60 for a 3 computer d/l and is worth every penny! Totally transparent until you need it. Never had a false alert, scans while the computer is idle, updates in the background, firewall, anti-spyware. I suppose other AV's have similar features but this works for me. If you look around you may be able to find it cheaper than $60.
I've purchased Norton years ago and hated it. Seems it messed up my OS, especially the *.DLL's which were more of a pain than some virus. I think Norton got this one right.