From MS website:
So how did you guys do a clean install? Or are you not activating the product????
Why can't I activate Windows?
If you received a message saying that Windows couldn't be activated, here are a few reasons why it might not have worked, and some things you can try to fix the problem. For additional help with the activation process,
contact support.
You upgraded to Windows 8, but didn't have a previous version of Windows installed (error 0xC004F061) If you see error 0xC004F061 when you try to activate Windows 8, it means that you're using a product key for an upgrade version of Windows 8 and a previous version of Windows wasn't on your PC when Windows 8 was installed. To install an upgrade version of Windows 8, you must already have Windows 7, Windows Vista, or Windows XP installed on your PC.
If you formatted the drive before the upgrade version of Windows 8 was installed, you won't be able to use your upgrade product key to activate Windows 8. To activate Windows 8, you'll need to install your previous version of Windows, and then reinstall Windows 8.