I switched to Avast! on my old XP rig a little while back and have really liked it. Absolutely no reason to switch so far. For a free app, that's pretty good. Both of my parents also run it on their home Vista desktops with (not insignificantly) zero complaints.
I've also used AVG Free with mostly good results.
I have personally used McAfee and, more extensively, Norton (*ugh*), with much more mixed results. Though it's been a few years since my last use of either, I feel fairly confident when urging folks to avoid these popularized/commercialized brands of AV. The system resource consumption on these beasts was (is?) well beyond excessive, which makes the average protection quality offered look and feel even worse than it is, IMO.
I've heard very good things about Kaspersky and NOD32 over the years... never tried either.
Microsoft has improved their antivirus products drastically in the last few years, to the point where OneCare (and now Security Essentials) is generally considered to be quite reliable and a very good free option after once being an absolute joke.
One other free product worth mentioning is Avira AntiVir, which is known for very strong detection rates, but less user friendly controls/interface than some other free options.
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