Just install it from scratch on the new machine with the same key, copy your files over and delete the source installation. Don't ever move it or move disks - it will shit a brick.
The other point that I raised was that I suspect I'd loose my media centre upgrade - well I assume I would, which frightens me a little, as it was the main reason for grabbing Win 8 in the first place. But I digress.
Instructions:
http://www.eightforums.com/tutorials/18309-windows-8-upgrade...