One way Microsoft attempts to protect it’s software from piracy is via product keys. In the case of Vista, this backfired spectacularly but If your PC came already installed with Windows XP, you should have received a recovery disk with the product code clearly visible. If you didn’t, or if you had to install XP yourself but have lost the product key, then you won’t be able to reinstall it without the key.
Annoyingly, although the Product Key is stored on your computer, Windows scrambles it so that you can’t read it. One handy solution is Magical Jelly Bean Keyfinder to recover and decipher the product key for you. However, if Windows XP came packaged with your computer and is not the retail version (with box), the key recovered by these tools may not be that of your own machine but the OEM’s bulk install key.
In which case, it’s worth trying RockXP which may be able to recover your individual keys although there’s no guarantee it can help. The advantage of this program however is that it can also recover product keys for many other Microsoft programs including MS Outlook and MS Office.