I believe he's written down the Product Number... not the license code.
With previous versions of Windows this acceptable as the Product and license code were the same.
How-ever with XP onwards (and maybe W 2K... not sure my 27 yr old memory isn't too good) the license code would be converted into a product code when entered. Unfortunately you can't use a product code to activate Windows XP.
Here's the next thing though... MS licensing states that any officially licensed version of Windows XP must have a license sticker on the side or top of the computer (or underside if a laptop). If it doesn't then naughty naughty. Being a MCSE, MCICT, MCSA (could also list aload of MCPs) I can't and wont help you find a license key for an unlicensed version.
You may be lucky and be able to buy an OEM license and media disc of ARIA.co.uk
The other option is a downgrade license where you buy a Vista/W7 license and downgrade to XP but I have a feeling they have either stopped that or are about to.