The Windows 8 Pro activation of additional features is a bit dumb. Instead checking if the entered key is already used it just checks if the key is valid and then upgrades the Windows 8 Pro version to Windows 8 Pro with Media Center. Windows tries to activate the new key after the next reboot. If the key is already used your Windows isn’t activated anymore even with the bought Windows 8 Pro key. And the best of all you can’t go back. No system restore point or any other option. Just a reinstall of Windows helps.
Problem was the free MediaCenter (for a limited time) and only one key per email. So I used the same email for my desktop pc which I already used for my laptop. So I got the same key. It’s easy to get a new key with another email address but you have to wait up to three or more days until you get the new key.
Well, I got a new key and so I could reactivate Windows 8 again.