"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."