Below is a complete, informative essay explaining what afxwin.h is, why you shouldn't download it separately, and the correct way to obtain and use it.
– MFC headers and libraries are tied to specific versions of Visual Studio and the Windows SDK. An afxwin.h file from an old Visual C++ 6.0 installation will not work correctly with a modern Visual Studio 2022 compiler due to changes in calling conventions, #pragma directives, and internal structures. afxwin.h download