I found a link in another thread to
this article:
1. Open regedit by pressing Windows-q, entering regedit and selecting the result from the list of hits.
2. Navigate to HKEY_LOCAL_MACHINE/Software/Microsoft/Windows/CurrentVersion/Setup/OOBE/
3. Change MediaBootInstall from 1 to 0
4. Go back to the start screen and enter cmd there.
5. Right-click Command Prompt and select to run it as administrator.
6. Type slmgr /rearm on the command line and hit enter.
7. Reboot Windows now.
I followed the steps listed, and rebooted. I didn't need to activate my copy of Windows Pro 8, it seems to have "unblocked the pipes" and straight after reboot I see "Windows is activated" in:
Control Panel > System and Security > Action Centre > Windows Activation
Click to expand...