Enable Windows 8.1 Using the Windows 8 Key
Enable Windows 8.1 Using the Windows 8 Key
The retail Windows 8.1 installation requires you to enter the product key. OEM and retail ISOs are identical, with the exception of VL which uses GVLK by default.
It has consistently been this way with Windows 8. Windows 7 and Vista allowed you to bypass the key, but not with Windows 8.