I've never really bothered with antivirus software or firewalls before, and it's never been a problem, but since I just invested in a nice new Windows box, I figured I'd get some extra protection.
What sort of features should I expect out of an antivirus program? I mean, they all obviously scan the HDD for viruses, so what else? Do some update their virus definitions more quickly than others? Or I suppose some programs are less demanding on system resources than others? Apparently there are testing agencies that compare and rate virus software; should I consult one of those?
Note: Please do not tell me what you think is "the best" antivirus software. I am looking for objective criteria that I can use to decide what is best for me. I imagine that a good answer would actually be OS-agnostic, unless there is some particular feature that I should look for on Windows (like some kind of registry scan for instance).