Just out of curiosity I want to know what language was used to write Windows itself.
We use almost entirely C, C++, and C# for Windows. Some areas of code are hand tuned/hand written assembly.
Well, the Windows kernel is still almost entirely C. However, there are different aspects that use C++, say some parts of the shell and so forth. The core is C. Other languages, perhaps even .NET might be used in some corners for certain applications but C is the OS language of choice in Windows.