Sometimes, you just need to do a clean install. Unfortunately, the Windows 8 Upgrade doesn't always allow for that, throwing you an error when you try to activate after a clean install. Reader uncommoner shows us a workaround for this issue.
If you do a clean install using the Windows 8 Upgrade Assistant, you should be fine—but if you've already formatted your drive or you're moving to a new drive, you can't do a "clean install" without installing an old version of Windows first. It'll let you install Windows 8 cleanly, but when you go to activate, you get an error 0x8007007B, saying your product key can only be used for upgrading.
If you get that error, here's how to fix it:
Press the Windows key and type regedit. Press enter to open the Registry Editor.
Navigate toHKEY_LOCAL_MACHINE/Software/Microsoft/Windows/CurrentVersion/Setup/OOBE/and double-click on the MediabootInstall key in the right pane.
Change the key's value from 1 to 0.
Exit the Registry Editor, press the Windows key again, and type cmd. Right-click on the Command Prompt icon and run it as an administrator.
Type slmgr /rearm and press Enter.
When you get back into Windows, you should be able to run the Activation utility and activate Windows as normal, without getting an error. Obviously, you could use this trick for evil, but it has its legitimate place too—if, say, you're upgrading your hard drive and want to do a fresh install on it, or if you formatted your drive before upgrading.
We haven't had a chance to test it ourselves, but it's been well documented around the net, so we're confident it should work for you if you're getting this particular error. If you give it a shot, let us know how it works for you in the discussions below! Thanks for the tip, uncommoner!