Windows NT4 had a 9x interface. Pre-4 NT windows versions had a Win3.1 interface.
Windows XP was practically based on Windows 2000, but the main 'difference' was that they ported the whole up-to-date directX library to it. Perhaps also some other backwards compatible features came with it and some server-specific functions were left out in the home version, but I don't know the details of that.
Perhaps those backward compatibility features rendered it somewhat unreliable but the main reason I think is badly written (often pre-installed) virusscanners/firewalls that clogged the system most often over time.
The biggest XP issue was IE. With 2000 I could install or uninstall IE, XP it was forced in to win the court case and therefore no matter how hard you worked at securing XP IE was a big hole straight to the OS Kernel that was easily exploited. XP got better after a few years and three service packs but I still can't get an XP box (or win7) to go 9 months of daily use without needing a reboot (even if you exclude patching).
The second issue was lack of control of virtual memory. I wasted one of my technet cases asking Microsoft why it was still swapping at 50% when I had set the reg key to swap at 99%, they told me they dropped support of that. So when my W2k workstation could easily run 3-4 virtual guests XP fell over at 1-2.
The inability to repeatedly use USB or resume were two other big issues I had. This was from a clean install without third party virus stuff either, those didn't start to get bad until later when they all tried to fix IE.
I saw many more XP BSOD's than I ever saw Amiga GURU's. Win2k I ran the beta for 9 months without a reboot, suspending and resuming my laptop at home, the office and client sites.
