Microsoft does not allow activation of clean installations using Windows 8 upgrade keys...
Microsoft offered super cheap Windows 8 Pro upgrade keys and users have been asking whether they can perform a clean installation and activate Windows 8 using the upgrade product key, and the answer is "No" and "Yes" !!
Confused!, well Microsoft will detect whether you upgraded or clean installed and block users from activating using the upgrade product key showing the error as below. But, users can still get the product key working by doing a little registry hack which will enable them to activate - this is not fair (or legal I guess).
"The following failure occurred while trying to use the product key:
The Software Licensing Service determined that this specified product key can only be used for upgrading, not for clean installs.