You can install Windows on your system but to activate it you will need an activation key that will be provided to you only after purchasing the Licence. Even Windows is allowed to be used by new users but these Windows come with a Product Key that will expire in a few months. Microsoft allows users to access its products and that's even without a license but these products are allowed to be used for only a few days or for a few months before the product will stop working and will ask you to purchase the license. If you are using any version of Windows, whether it's Windows XP, 7, 8, Windows Server 2008 or 2012 and you don't have the complete license for your Windows then you will not be allowed to use the product after a period of time. In this article you will learn how to change the Product Key of your system.