nixpkgs/pkgs/development/compilers/visual-c++/test/hello.c

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