What should I look for in an antivirus program?

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).

To me it needs to stay out of the way, honestly.There's nothing worse than an AV that slows down a system as much as the viruses its supposed to protect against (older versions of norton come to mind), or decides to announce loudly that it has updated (I can't remember which one this was).

I like AVs that do what they say on the tin - I want an antivirus, not a 'web site security checker' or 'suite' or firewall. If i need those i can install it myself.

Most importantly, it needs to work!

See similar: https://superuser.com/questions/2084/what-anti-virus-software-is-easiest-to-novice-users-to-keep-up-with-and-not-get.

Note how it turned into "My favorite is...". :)

The main criteria are going to be speed and accuracy when adding to their definition lists and detection types. Which varies wildly from product to product, month to month, year to year. A lot of it is "who found it first", and "Who added it last".

Personal choice and experience play a big part in it as well.

My suggestions:

You'll want to look into what different types of anti-malware Detection Systems they have implemented.

For a network of computers, Centralized Management and Alerting may trump many other factors.

Their Customer Service can quickly change your opinion the first time you run into trouble.

Ease-of-Use and the UI would be huge, but since this is not OS-specific it can't be considered too much, as it can also vary greatly from platform to platform (for the same product line).