The NT kernel is the most beautiful piece of code I've seen. Dave Cutler and team wrote some very, very elegant code that anyone (even if you're not a kernel hacker) can understand. If I need 'code inspiration', I spend some time looking through their code.
Now, the NT kernel itself doesn't have source out there but for folks who are students, you should be able to get your hands on the Windows Researh Kernel (http://www.microsoft.com/resources/sharedsource/windowsacade...). This has all the good stuff (and is much easier to build too)