Since first public news of Windows 7's development back in Oct 2007, we heard about MinWin -- element that sounds like some kind of portable Windows kernel. At PDC09, Mark Russinovich explained to developers “it's a way to graft onto Windows some semblance of architectural layering it should have had, if its architects in 1980s had any foresight into how Windows would be used thirty years later. It enables current and future Microsoft developers to evolve new configurations of operating system, without having to rewrite core services or worry about breaking dependencies between those services and upper-level APIs. "If you look back at the evolution of Windows, it's evolved very organically, where components are added to the system and features are added to the system without, in the past, any real focus on architecture or layering," Russinovich explained. "And that's led us to do some hacks with Windows, when we want to make small footprint versions of Windows like Server Core, or Embedded Windows, or Windows PE -- pre-installation environment. What we do [instead] is take full Windows, and start pulling pieces off of it. The problem with that is, pieces that’re left sometimes have dependencies out to pieces that we've removed. And we don't really understand those dependencies."
More info: Mark Russinovich on MinWin