Microsoft used to make two GUI operating systems: Windows and Windows NT. The former is intended for the consumer market and is meant to be optimally user-friendly; the latter is intended for a business with servers and workstations, and is consequently more powerful and far less prone to crash. Since the 1995 release, Windows has been designated with a year (e.g., Windows 98) while NT has been given a more conventional version number (e.g., NT 4.x). However, Windows 2000, aka Win2K, is actually a complete redesign of Windows that has much more in common with NT. This has led to the oddity of having simultaneous releases of Windows 2000 and Windows Millennium Edition, which is a last-gasp update of Windows 98 for older PCs and people who need total backward compatability with MS-DOS.

Anyway, the unofficial name NT2K is a way of acknowledging Windows 2000's real parentage. I can affirm that this is significant; Win2K is qualitatively less crashy and restrictive than older Windows.