Well, I dont think this computerhope.com has a policy of helping people with serial codes and product keys.
1) If you have a original manufacturer PC eg. DELL, HP whatever, phone the helpdesk.
2) If you purchased the XP box, then you most definately will have a product key inside the cardboard holder.
3) If it is some kind of clone pc with windows XP preinstalled, then look for a file called on WINNT.SIF you will find a product key in there. That is asuming that it was not deleted during some script.