I've been using Norton's Antivirus 2002 in conjuction with XP every since I got the two together. I just bought SystemWorks 2002, so it will be to have the integration of all the various services.
As for Norton AV itself, it's actually very good. I expected a resource hog but it was actually quite light on the resource and very high on protection. I've tried a few of the free ones out (AVG, Inoculate etc) and although they were certainly functional, I never really felt as protected. I think, at least for peace of mind, Norton AV is the way to go. The very frequent updates released are reassuring.
AVG would come a close second. Very light on resources, very easy to install and perhaps not as bloated as Norton AV. For those who don't want to dish out money on Norton AV or SystemWorks, AVG would be my second choice - and it's completely free. Paradise!.