I had a great video forwarded onto me by another IT Pro. Microsoft has a presentation by Mark Russinovich on Windows Security Boundaries. This is a great resource to learn how Microsoft approaches security boundaries in their operating systems. Some of the security boundaries that are covered include physical machines, system virtual machines, and user sessions. Mark also goes in to processes, patch guard, and kernel-mode signing. As always Mark does a great job of breaking down security using some of his System Internals Tools. The presentation is about an hour and fifteen minutes long. Personally I think it is well worth your time if you want understand operating system security better.
Windows Security Boundaries On Tech Net