I used a bootleg version of Windows XP, which serves me all right, but I can't get any updates without some sort of validation, and I'm stuck in service pack 2. I do, however, have a legit version of Windows XP now (brother's XP code for a computer he no longer uses), and I figured I'd replace my bootleg CD key with the legit one and go for them Windows updates. Anyone got any tips for this?

you can try DL'ing the Windows XP Service Pack 3 from here:

Windows XP Service Pack 3

it's got most of the up to date security fixes, and it'll install fine, whether your WinXP is genuine or bootleg.

This might work. Don't forget to back up your registry beforehand.
http://pcsupport.about.com/od/productkeysa...changexpkey.htm