I wouldn’t have believed it, but it actually possible to install Windows version 1 and upgrade through every major version of Windows all the way to Windows 7. This video has some great details and screen shots of the process and details some of the major differences and the items that are kept between upgrades. Kudos to the guy who actually had all the time needed to walk through this monster process. I should also commend Microsoft on 20 years of compatible upgrades.