I think technically Win2K lived in the branch known as 'NT' so there was Windows NT, W2K, and then merged back at the Vista point (and made usable by Win7) point. The timeline as presented would be accurate for a 'home' desktop user.
And that was:
* NT 3.1 - Had Windows 3.x type UI
* NT 3.5 - Had Windows 3.x type UI
* NT 3.51 - Had Windows 3.x type UI
* NT 4 - Had Windows 9x type UI
* Windows 2000 (NT 5)
* Windows XP (NT 5.1 - XP x64 was 5.2 though)
* Windows Server 2003/R2 (NT 5.2)
* Windows Vista/Server 2008 (NT 6.0)
* Windows 7/Server 2008 R2 (NT 6.1)
* Windows 8/Server 2012 (NT 6.2)
I remember NT 4 was used on the main server(s) at my highschool.
I think it's safe to say, XP was the first consumer/"home" user version. Though, Windows 2000 (Professional) was used in corporate environments.
