Any help or advice, please?

I have a (legal, licensed) copy of Windows XP on an 18 month old HP laptop.
Unfortunately, I now need to re-install Windows XP and I have lost my Windows
XP recovery CD. To add to my problems, HP no longer sells the recovery CD for
my particular computer. Would you please advise me of how I can legally use
my existing product key to re-install Windows XP? Thanks!