I'm runing "avast!" under Vista x64 and it is working very nicely. The Office/Mail integration seems quite happy with Office 2007 as well.
Under XP x86 I used to run Norton AV 2006, which was ok, if a bit of a pig (way too much stuff I didn't need or want). I'd never gotten around to trying anything else, but given all Symantec's bitching about getting special kernel hooks and not having anything I could use right away for x64 I figured I would ... and I like what I'm seeing.
One nice thing with Vista is that the integrted blocking firewall, while basic at a feature level, also isn't bloated with all the worthless broken shit that crept into ZoneAlarm over the years (ad-blocking that blocked everything BUT ads for one!) ...
So /wave Norton and /wave ZoneAlarm ... all I wanted was basic functionality, not the take over of my entire systems ...