Probably a mixture of languages
Some assembler is exected for the low-level parts
Most of the kernel is most likely in C.
Most tools on top of it, that are integral part of XP are probably C/C++
with somethings like MFC and ATL.
As of yet, I don't expect much to be in a .NET language, expecially not if
it is kernel related.
Its probably not too different from other OSses, like Linux, *BSD, ...
- Joris