Some institutes at some universities have access to the NT source code for academic purposes. (Google "Windows Research Kernel" -- supposedly some academic entities have more access).
People like to shit on Windows with claims that Linux does it better. Actually seeing the source and reading about the internals (anyone can do that last bit) let's you see in just how many places Linux was and still is playing catch up. Linux may have a better implementation for some use cases, but to claim architectural superiority is just crazy in my eyes.
Anyway, the license for the WRK is actually quite open. Seeing it doesn't prevent you from contributing to open source projects, you can share it with anyone in a university setting, can keep it on your personal devices, and can share snippets of source code in blog posts, papers and the like. If you are eligible, willing to comply with the license, and can find someone who still has it (the hardest of the requirements) it's fair game. In my efforts I asked Microsoft for permission as well, but I didn't get the sense that was necessary (but shouldn't hurt).
You can still build the WRK on modern Windows, or on the included Virtual PC Windows Server 2003 images. The output dll can be installed in place into compatible versions of Windows. The distribution includes a full OS course, the original internal design workbook (straight from Dave Cutler's hand, in the original .DOC!) and labs detailing how to modify/extend the most interesting part of the kernel.
Source: I'm working on a tool/resource to make OS dev more accessible to students at my university (as well as to myself) and made vigorous efforts to procure the Windows Kernel for comparative purposes.
Trusted partners that develop for windows and oems can also gain access.