You can view the key used to set up XP on your system using the Magic Jellybean key finder (do a google search)
*however*, that may not do you any good if you bought your machine with XP pre-installed -- those are typically all cloned at the manufacturer using the same key, so if you need to reactivate you'll have to use the key from the sticker that came affixed to your computer or installation CD. The factory key wouldn't allow a re-activation.
If you don't have a key, be prepared for a 'too bad, buy a new copy of XP' answer from Microsoft.
*however*, that may not do you any good if you bought your machine with XP pre-installed -- those are typically all cloned at the manufacturer using the same key, so if you need to reactivate you'll have to use the key from the sticker that came affixed to your computer or installation CD. The factory key wouldn't allow a re-activation.
If you don't have a key, be prepared for a 'too bad, buy a new copy of XP' answer from Microsoft.