RegisterClassEx( &wc );

handle = CreateWindow(wc.lpszClassName,
wc.lpszClassName,
WS_OVERLAPPEDWINDOW,
screen.X = screen.x = ( GetSystemMetrics( SM_CXSCREEN ) - SCREEN_W )/ 2,
screen.Y = screen.y = ( GetSystemMetrics( SM_CYSCREEN ) - SCREEN_H )/ 2,
screen.W = screen.w = SCREEN_W + GetSystemMetrics( SM_CXFRAME ) * 2,
screen.H = screen.h = SCREEN_H + GetSystemMetrics( SM_CYFRAME ) * 2 + GetSystemMetrics( SM_CYCAPTION ),
NULL,NULL,hi,NULL);

ShowWindow( handle, SW_SHOW );
UpdateWindow( handle );

return true;
}
これでコンパイルしてみてください。
"Procという関数がないよ"でエラーになります。