The Windows API is an array of functions that can be called the same from C as from C++. Microsoft provides several object oriented wrappers for this API, and it's very rare that any significantly large application is not calling the API over some kind of wrapper.
No, I think it's C. It's written in C and was (at least initially) designed to be called from C code. I think #ifdef __cplusplus followed by extern "C" doesn't make a C API C/C++. Also the fact that there are several Microsoft and third-party wrappers out there doesn't make it any less C, at least in my opinion.
