As mentioned before,in WinNT and Win2k, all I/O must be doen thru Windows using APIs.

This is not entirely true.
You can write a DOS program which messes directly with, say, the IO port registers for the parallel port and it can still work when run under NT.
I've done it, and was quite surprised to find that it did work on NT!

Windows NT (and, presumably, 2000) "humours" such DOS programs into thinking that they're hacking right in there straight to the hardware when, in fact, Windows is sneakily "grabbing" those accesses and diverting them through all its driver layers.
Unfortunately, this is not foolproof and can lead to weird results...!