AVG Free supports all those operating systems[*edit* Oops, it doesn't support 2003 server for free]. I only use it because it's free, though. If I paid for one I'd use NOD32. Check the licenses first.

I've had a little difficulty with AVG Free version 8, but I think they've ironed out most of the problems by now. Build 138 seems good.

I have Clamwin on my portable apps drive, but I've never used it or even read about users' experiences with it. Does anyone here use it?