Someone on this thread mentioned that NT was based on Mach.  Just thought I'd point out that this is not true.  Mach was a research project at Carnegie Mellon, and while it is true that Richard Rashid was hired by Microsoft at one point, he was not involved really in the development of NT aside from some minor research related to paging the kernel.

The Windows NT kernel was designed from scratch by Dave Cutler and his team at Microsoft, most of whom came from Digital Equipment Corp and were involved with the development of VMS (another Cutler design.)

This is a long held misconception, perhaps related to the fact that both operating system kernels (Mach and NT) were billed as microkernels, when in fact both kernels are better classified as "hybrid kernels." 

But aside from being modern operating systems, they are not related.

References:
"Showstopper!"  by G. Pascal Zachary
"Inside Windows NT" David Solomon