Windows.h is a Windows-specific header file for the C programming language which contains declarations for all of the functions in the Windows API, all the common macros used by Windows programmers, and all the data types used by the various functions and subsystems. It defines a very large number of Windows specific functions that can be used in C. The Win32 API can be added to a C programming project by including the <windows.h> header file and linking to the appropriate libraries.

#include <windows.h>
 
int WINAPI WinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance,
                       LPSTR lpCmdLine, int nCmdShow)
{
 
     MessageBox(NULL, "Hello World!", "Sourcecodeera.com", MB_OK);
     
     return 0;
}