They key is encrypted in the registry, you'll need to download a program that will show it to you in plain text. Bear in mind that XP requires you to register it, so you can't use it on more than one machine for more than a few weeks.http://www.magicaljellybean.com/keyfinder.shtml
is the best utility I've found for pulling out the number. I've used it myself when I was going to reinstall and didn't have the sleeve anymore.
Google is your friend - "xp key finder" came up with a number of links.