We've got McAfee Active Virus Defense (blurgh, you Americans and your butchering of my language), comprising VirusScan Enterprise 7.0, Groupshield, ePO, etc., and that installs a couple of filesystem-level drivers (specifically I'm thinking of NAIFSREC.SYS). It's the only way you can have on-access scanning without killing the performance of the machine.
Sophos and whatever Kaspersky Labs product is called do something similar.
Norton Antivirus home edition is a horrible piece of cack. The corporate version is much more reasonable. I'm not particularly enamoured of the whole Live* thing, though. The engine is more-or-less the same, but the corporate edition is clearly aimed at grownups, whereas the home version (Norton Antivirus 2003?) is clearly aimed at the under-10s market segment.