I don't think you'll find it in the registry directly but there are some tools that will find it for you. I found one using google - haven't tried it though so scan with av program before using it.
http://www.updatexp.com/xp-key.html
If you can't boot to windows, you might have to contact hp with your purchase information to get this sorted. Even if you had the key from the side of the machine, sometimes these don't actually work anyway. It's because the systems are mass produced using a different key and this is just a license number that they are authorised to give out with the clients.