I use
http://www.avira.com/en/support-for-free just switched to it lately seems to be very good, had to ditch AVG free as it was getting too invasive, bloated and I don't agree with apps that install plugins to firefox without option. Did a full scan with avira and it picked up 2 problems AVG never did.
I've never had to reinstall XP on my 5 year old rig and its running as good as new, actually better, I've done that many tweaks to it I've forgotten most of them here's a good place to start...
http://freepctech.com/pc/xp/xpindex.shtml
Mind you I could almost run without a virus scanner as I only use Firefox with NoScript, AdBlock Plus, Ghostery and BetterPrivacy Add-ons. I also immunise with spybot search and destroy and spyware blaster, so there's not much that's going to get through my .net, oh and I also use peerblocker. :)
Also for window dressing xpize is great.