You don't need to use C if you want to call Win32 functions, although the calling convention is based on C. The stable API is not the user-kernel boundary but instead a set of DLLs (kernel32, user32, gdi32, etc.), which makes sense if you look at the history: Windows started out as a GUI on top of DOS. The actual system call mechanism is very different between the DOS-based lineage and the NT-based ones, but the Win32 API remains nearly the same.