How-to: Windows 8 Professional Upgrade install and activation on system without OS installed

Using this info you should be able to do a clean install of the Windows 8 Upgrade on a system without an Operating system.

• Open regedit by pressing Windows-q, entering regedit and selecting the result from the list of hits.
• Navigate to HKEY_LOCAL_MACHINE/Software/Microsoft/Windows/CurrentVersion/Setup/OOBE/
• Change MediaBootInstall from 1 to 0
• Go back to the start screen and enter cmd there.
• Right-click Command Prompt and select to run it as administrator.
• Type slmgr /rearm on the command line and hit enter.
• Reboot Windows now.
• Run the activation utility afterwards, enter your product key to activate Windows.

This is the same method that worked for Windows 7.