The core Windows OS is actually very cool, IMO. Pity it's closed source. Some time ago, I did some research on Windows 2000 and wrote a lot of it down on the Windows 2000 Wikipedia article. I later split it to the "Architecture of Windows 2000", which interestingly (and correctly) has been renamed to "Architecture of Windows NT" [1]
What you are referring to, however, is in user mode, in the environment subsystem. There is actually a few subsystems - the one you are talking about is the POSIX subsystem. I believe it's actually still there in the latest versions (might be wrong about that).