Windows is not written in C++, even if Microsoft did embrace C++ quite a bit with the infamous MFC and so on. Windows was originally written using C and assembly language, using the Pascal calling convention for efficiency. It was originally a layer on top DOS which was written in assembly language. Being a "layer" or not was the subject of a very complex federal antitrust case back in the 90s. You can read "Undocumented DOS" and "Undocumented Windows" for some weird kind of technical investigation thriller which I found weirdly fascinating back in the day.