Windows 2000 SP3 was felt really nice compared to constant crashing with 95/98 and XP's ugly UI and again increased crashes. But if you try 2000 now, it's clumsy and it was only SP3 that was finally stable.

XP could be easily improved by just switching to classic UI style and any wallpaper other than the default teletubbyland. Still I think 7 was the first Windows that didn't need reboot at least once per day and would't give blue screens.