Suppose back in 1991 you'd installed MS-DOS 5.0 and the classid iD game DOOM 2 on a machine which -- via the magic of thought experiments and time machines -- was far enough advanced at the time that eventually it would support Windows 7 when it was released 19 years later. Suppose further that you then upgraded (not fresh-installed) every update of Windows that came out since then, in turn. (Not counting ME, but then, who does?). Think Doom II would still work by the time you got to Windows 7? Well, blogger Andy did just this (using a modern machine and VMware, instead of a time machine), and shared the results:
Yes! Doom II still works -- an incredible result, given all these upgrades, changes in file systems and processor bit-widths. Even other old MS-DOS programs, like Reversi and Card File can be found on the repeatedly-upgraded system. There are a lot of things to complain about regarding Windows -- not least the ever-increasing time it takes to update Windows (2 hours for 7.0!) as illustrated in the video -- but that's still a pretty amazing achievement.
The video is also a fascinating look at how the Windows UI and especially the install procedure has changed over the years.
Andy's Tech Experiments Blog: Chain of fools : Upgrading through every version of Windows (via Kottke)