Looks like .NET is not that import after all:
Microsoft appears to have concentrated their development effort in Vista on native code development. (...) Vista desktop shell is not based on the .NET runtime. The only conclusion that can be made from these results is that between PDC 2003 and the release of Vista Beta 1 Microsoft has decided that it is better to use native code for the operating system, than to use the .NET framework.
The author concludes that "(...) Microsoft has lost its confidence in .NET. They implement very little of their own code using .NET."