While the Microsoft Windows 8 Upgrade Key is intended only for upgrades from earlier Windows versions to Windows 8, there could be valid genuine reasons when a user would like to do a clean installation of Windows 8. If a user tries to do a clean install activation using the upgrade key they are shown the error below and no activation occurs. Luckily, if a user owns valid license key for previous Windows version they can use a simple registry hack (I assume legally) which enables them to bypass this activation error message.

Invalid Product Key

"The following failure occurred while trying to use the product key:
Code:
0xC004F061

Description:
The Software Licensing Service determined that this specified product key can only be used for upgrading, not for clean installs."