There are times when you have to do a clean install of your Windows 8 but if you have just an upgrade key you need to make couple of extra hoops before you can activate the new install. The upgrade key doesn't prevent you installing to a clean disk but when you try to activate, you get an error 0x8007007B, saying your product key can only be used for upgrading. Another fine example how Microsoft makes things complicated for legitimate users.
Fortunately there's a way to fix that issue as Lifehacker's article tells:
- Open the Registry Editor (Win + R, type regedit).
- Navigate to "HKEY_LOCAL_MACHINE/Software/Microsoft/Windows/CurrentVersion/Setup/OOBE/" and change the MediabootInstall key's value from 1 to 0.
- Open the Command Prompt (Win + R, type cmd). Right-click on the Command Prompt icon and run it as an administrator.
- Type slmgr -rearm and press Enter.
- Reboot Windows.
After that is done and you get back into Windows, you should be able to run the Activation utility and activate Windows as normal, without getting an error.
Apparently you can also call Microsoft Support and they will walk you through the proper way doing this because they understand you may have bought a new drive. There is a dialog you can get to in system tools where it will ask you for a numerical code. The support personnel will give you the number, you click "OK" after typing it in, then go the activation again and it works. This process is likely doing the work around mention above, but through an approved administrative process.
By the way, restarting Windows 8 is most easily done by left-clicking once on an empty spot on the desktop and holding Alt + F4).