Windows 8.1 is the "real" Windows OS that runs on PCs and tablets; it includes backward compatibility with most earlier Windows software. programs and games that were designed to run on Windows 7, Vista, and XP. By contrast, Windows RT is a stripped-down version of Windows 8.1 Prior to Windows 7, purchasing an OEM System Builder license for your own PC was perfectly fine. In fact, there are legitimate drawbacks to this cheaper license