Let me break it down for you...
There were multiple versions of Windows NT.
Windows NT v3.0, v3.5 and of course v4.0.
Now here's where you need to pay attention.
Windows 2000 = Windows NT v5.0
Windows XP = Windows NT v5.1
This is pasted directly from the command prompt window you can get from running the "cmd" command in the RUN dialog in the Start menu:
"Microsoft Windows XP [Version 5.1.2600]"
The ".2600" is a BUILD number. (Many programs are rebuilt and recompiled several times depending on certain circumstances.)
Windows 2000 and Windows XP are both based on the Windows NT Kernel. (The core of the operating system.) Does this make more sense to you now?
There ya go. Either way... Wanna know the truth about windows? Well I can't put the image here for some reason so here is a link to it. (Why do I get a page error when I click the button to insert an image? Something wrong with emutalk?)
http://stormhaven.servehttp.com/wincement.jpg