I have an important warring; using free Windows Media Center Keys will render Windows 8.1 not activated. I know everyone wants to keep using their free copy of Windows Media Center for Windows 8 but do not attempt to use that key to upgrade to Media Center with Windows 8.1. The Add Features to Windows 8.1 option in Control Panel will take the free Windows Media Center and it will install Media Center but as soon as Windows restarts it will not be activated. I did not create a System Restore point before trying the upgrade so I could not do anything but reinstall Windows 8.1.
Now if you installed Windows 8 used the free Windows Media Center key and got that to activate and then upgraded to Windows 8.1 that may work but I have not tried it yet to verify.
So just some warnings and ideas for those of you wanting to clean install Windows 8.1 but keep Windows Media Center. I suggest either trying my suggestion or just pay the $10 to get Windows Media Canter after you clean install Windows 8.1.