I thought Windows 2000 was an operating system for servers? It morphed into Windows NT.
Though that leaves the question why DOS was good, Windows 3.0 was fine, Windows 95 was fine. 98 was much better. XP was a huge pain in the ass since it had nearly no backwards compatibility for programs, meaning none of my games would run. Vista was basically XP with some improvements, better backwards capability, and that terrible waste of RAM. 7 I haven't had enough experience with, but I like the new setup for the toolbar.
The thing that gets me is that everyone heralds the end of Windows with every new operating system, when usually most of the errors are either user based or very small. I think the largest issue I've had is the backwards capability of XP, while Vista RAM thing was annoying but not dooming.
And why does everyone sit around for the downfall of Microsoft? Um, Mac isn't better and there's a lot of externalities that are good to having only one or two major OSes.