Full, official and legal version of the software - download directly from Microsoft and authenticate online.
The OEM version is limited - it permanently binds to one PC on first activation, it does not come with Microsofts free phone support and you can't use it to upgrade from Windows 7 or 8.1 - only fresh install. We recommend purchasing the retail version that is free of any restrictions, especially if plan to upgrade or change your PC later and still use our key.