If you want to retain Windows XP, go to:
Keyfinder | Magical Jelly Bean and download the freeware version, which will tell you your product key. Write it down, and then ask any of your friends with computers whether you can borrow (or have a burnt copy of) their OEM XP CD. The CD isn't what constitutes the license so you shouldn't have problems finding someone who'll lend/burn you a copy.
Then it's just a case of downloading and installing the missing hardware drivers, updating via service packs and MS Update, and installing any software you wish.