You mean desktop environment? Funnily enough, Windows started as one (http://en.m.wikipedia.org/wiki/Microsoft_Windows#Early_versi...). It was considered a "graphical operating system shell", what we'd call a "desktop environment" these days.

It is technically doable, but I'm not sure the Windows NT kernel is worth discarding entirely. It has some fancy tech in there and if need be it can emulate POSIX through "personalities" aka subsystems (http://en.m.wikipedia.org/wiki/Windows_Services_for_UNIX). Subsystems actually make it more flexible than Linux.