I noticed no one has mentiond AVG anti virus
http://free.grisoft.com/
At 12.45p mine kicks in and updates it's self.
Plus I've never had any problems and any nasties trying to get in get caught & what It can't heel it sends to the vault,
so when I close down the site i'm in. I goto the vault and get rid of it from there.
A programmer friend recomened it.
Another also free and recomende is (
http://www.pctools.com/en/ Spyware Doctor is a top-rated malware & spyware removal utility that detects,
removes and protects your PC from thousands of potential spyware, adware, trojans, keyloggers, spybots and tracking threats. Now with AntiVirus!)
I use this after going into all those huge galliery site, Where you always get all sorts of crap.
So a quick
INTELLI SCAN gets rid of most crap plus it's set to do a full scan every couple of days.
So both programs together work well for this XP user.
Be safe