There are often times when you need to install Windows 10, Windows 8.1 or Windows 8 for evaluation or testing in a virtual machine like VirtualBox for example. You may not want to activate it every time with your licensed product key that you use on a real machine. For that purpose, you can use generic keys from Microsoft, which will allow you to install the OS, but won't allow you to activate it. As long as you have an ISO image containing Windows Setup files you can install the OS using a generic key. If you used Windows 7, you might remember you could install it without a key. Generic keys for Windows 10, Windows 8.1 and Windows 8 serve the same purpose.