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.
"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."
- Right-click at the bottom left corner of the desktop screen, where we had the start menu earlier and Click "Command Prompt (Admin)".
- In command prompt window type "regedit" and press enter to launch registry editor.
- Navigate to HKEY_LOCAL_MACHINE/Software/Microsoft/Windows/CurrentVersion/Setup/OOBE/ key and change value of MediaBootInstall to 0.
- Close registry editor and return to command prompt window, now type-in slmgr /rearm and press enter. DO NOTE THE CAUTION BELOW (AT END OF THIS POST) WITH REGARDS TO THIS COMMAND.
- Reboot your computer and you should be able to activate Windows 8 normally.
On my test setup I used the slmgr /rearm twice and my Windows 8 activation got blocked with Error Code : 0xC004C008 - The activation server reported that the key has exceeded its activation limit. The only solution to this is to call Microsoft Windows Activation Support Desk and ask for an agent to fix this.