Nod32 from ESET gets my vote. Very light on resources, rated highly for the amount of stuff it catches and very few false positives. Not intrusive.

Had an annoying false positive, the an aspx program being run by Outlook Connector was identified as adware. Fixed by ESET within a couple of days. Tech support not quite as fast as I'd like, but when is it ever fast enough?

Solid program, I recommend it.