Well well just the other day I found my old Windows XP CD and unfortunately I have no idea where I placed the serial, that's why I was asking if there was a way...
Where did the disk come from? If it is still actively installed on another computer (i.e. you'll be transferring it and won't use it on the other one anymore), you can pull the key off that computer (see link below). If not, and you have a disc, you might possibly be able to get a new key issued (like in the link as well).