There will be times you need to have your Windows XP Product Key from your original Windows disk (probably when you’re calling Microsoft). Here is one product that only looks up your product key:
Go to the site for — Magical Jelly Bean Keyfinder
This next product looks up everything on your computer (use "Microsoft - Windows XP Professional" when you search the results page) including most serial numbers. The program is free for personal use — but not commercial or non-profit use. Well, if you’re a non-profit, see the bottom of the page whose link I’m giving you for instructions.