Far be it from me to disagree with Asyncritus. AVG and a combination of Ad-aware and/or Spybot has worked just fine for me, but my excursions into the seamy side of the Internet are pretty tame. I just can't see any justification for most people to pay for anti virus protection when something perfectly good is available for free. YMMV.
I believe that's the case, although I haven't seen a box running anything older than XP for quite a long while. I would be more than a little surprised to see that, truth be told -- it'd be like running across a Win3.1 box shortly after XP had launched.