Reinstall Windows? No need!
The video describes and shows what happens if you try to upgrade from Windows 1 to Windows 7 (hehe). I must say it's kind of impressive, although some things are not preserved when upgrading, such as color and theme settings. Still, being able to run old code and applications (as included in the first release) is great!
I'd love to see something similar with Mac OS X. I recognize it might be somewhat of a problem because of the PowerPC to Intel switch (and emulation of a PowerPC might be hard). Nevertheless, would still me nice to watch.
I never upgrade my version of Windows because of three reasons. First of all it is a security risk, because a fresh install is clean, and the current system setup might be infected without me knowing. I also like my system to be in top shape, and I find it hard to "clean" Windows. It's easier now than before (Win XP for example), but it's easier to reinstall. Last, but not least, I tend to upgrade my PC hardware every once in a while. Driver Hell is where you go to when you change hardware and not reinstall Windows.