I have downloaded and installed Windows 8.1 RTM version 9600 using TechNet Retail Subscription key. It was a Clean installation. The OS got activated but I am unable to activate WMC.I used the same product key of WMC supplied by Microsoft.
There was a long discussion regarding this issue at TechNet forum. You can go through the thread :-
You may get this fixed by phone activation. Or,you can use a different Windows Media Center key (if you have). That’s the easiest way to fix this annoying problem. Otherwise just wait till 18th October, when it will be released to the public.