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.

Looking back at what Xerox PARC and ETHZ were doing with their workstations, maybe that wouldn't have been much of an issue.

I don't think there's anything in the nature of C per se that's required for Windows. Any reasonably low-level compiled language would have done the same job; Pascal is potentially the nearest substitute. Windows had to construct its own executable and library formats anyway.