said by MrFixit1:
What with the Media Center key boondoggle , it would not surprise me if you had to have a validated qualifying install instead of just an old install with a " valid " key , so don't know if just throwing a partial XP or 7 on would work .
My experience was that the W7 install that was upgraded to W8 had to be activated before proceeding with the upgrade.
Simply having the old W7 install disc in a drive or offering up the old W7 key code were not viable options for me to upgrade to W8. I had to upgrade from an installed, activated verison of W7 in order to upgrade using the $39 digital download.