Error “This product key didn’t work. Please check it and try again” while activating a new Windows 8 installation.

This error can occur if you try to activate a clean Windows installation with an upgrade license key. If that’s the case there is an easy way to fix this:

• Open Registry Editor (regedit.exe)
• Navigate to 
  KEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Setup\OOBE
• Double click the key MediaBootInstall and change value from 1 to 0
  
  

• Open elevated Command Prompt (cmd > right click > Run as Administrator)
• type slmgr /rearm and press Enter
• Restart your computer

Activation should now work fine.

Windows 8