const int SCREEN_WIDTH = GetSystemMetrics(SM_CXSCREEN) - 1;
const int SCREEN_HEIGHT = GetSystemMetrics(SM_CXSCREEN) - 1;
...
    int x;
    HDC screen_device = GetDC(NULL);
    HDC copy = CreateCompatibleDC(screen_device);
    for (x = 0; x <= SCREEN_WIDTH; ++x) {
        for (int y = 0; y <= SCREEN_HEIGHT; ++y) {
            SetPixel(screen_device, x, y,  GetPixel(copy, SCREEN_WIDTH - x, y));
        }
    }
...