Well, the first question would be: Is it legal to take hold of the NT Kernel Source? If so, proceed to the second paragraph; if not, proceed to the third.
The first thing you'd probably ask is "Why NT Kernel Source?" The answer is simply, I want to make my own OS compatible with Windows. This is of course, all a hobby, and I'm not planning any large project.
Ignore this if it's possible to get an NT Kernel Source: "ReactOS" I'm pretty sure that it uses Window's source code itself, with a few minor exceptions, so I'd like to know how they got the code.