Is this a serious question? It has a kind of non-authentic quality about it.
Microsoft doesn't distribute Windows XP via download for end-users to purchase nor has authorized any other party. MS did offer trial versions of XP 64-bit for download at one time but you had to purchase the full version to convert it. You couldn't purchase a license online then receive the 'non-trial' product key via email, AFAIK.
TechNet and MSDN subscribers can download ISO images of Windows XP and most other MS products for development and testing purposes, but not personal use or redistribution. Microsoft provides each TechNet and MSDN subscriber with product installation keys, IIRC, but I'm not sure they can be activated.
Of course, you "can" download Windows XP from 'other' sources, but...