Have a Windows 8 Pro 64-bit laptop that upgraded to the 8.1 Preview. Cannot upgrade to 8.1 directly which is fine, however cannot remove the 8.1 Preview. It tells me the system is missing files in order to recover the system and to insert the Windows 8 DVD. Which is the same Windows 8 Pro 64-bit upgrade DVD used to install Windows 8. Tells me invalid disc. Do I have to start with clean OS install, or is there a fix?