procedure WndProc(hWnd,uMsg:LongWord;wParam,lParam:Longint) stdcall;
begin
  case uMsg of
    WM_DESTROY: PostQuitMessage(0);
  DefWindowProc(hWnd,uMsg,wParam,lParam);
end;

...
в заполнении WNDCLASSEX
  wc.lpfnWndProc:=@WndProc;

ShowWindow( hWnd, 0 );

const
  SW_HIDE = 0;
  SW_NORMAL = 1;

сделай
ShowWindow( hWnd, SW_NORMAL );
или 
ShowWindow( hWnd, 1 );

    if (!hWnd)  
    { 
        return false; 
    }