C / C++ / MFC

for (int i=0; i<number_of_windows; i++)
if (!InitInstance (hInstance, nCmdShow))
{
  return FALSE;
}

for (i=0; i<number_of_windows; i++)
while (GetMessage(&msg, NULL, 0, 0)) 
{
  if (!TranslateAccelerator(msg.hwnd, hAccelTable, &msg)) 
  {
    TranslateMessage(&msg);
    DispatchMessage(&msg);
  }
}

LONG lCount=1;

LONG lMan=0;
do
{
  while (GetMessage(&msg, NULL, 0, 0))
  {
    if (!TranslateAccelerator(msg.hwnd, hAccelTable, &msg))
    {
      TranslateMessage(&msg);
      DispatchMessage(&msg);
    }
  }
  InterlockedDecrement(&lCount); // a window has been closed; put a breakpoint here!!!
  InterlockedCompareExchange((void **)&lMan, (PVOID)1,
    (PVOID)lCount); // if lCount==lMan==0 then lMan=1
} while (!lMan);

case IDC_BUTTON:
  InitInstance(hInst, SW_SHOW); // new window
  InterlockedIncrement(&lCount);
  break;

HBITMAP hbmp1 = NULL, hbmp2 = NULL;
BITMAP bmp1, bmp2;
.
.
//load the bitmaps in hbmp1 and hbmp2
.
.
GetObject(hbmp1, sizeof(BITMAP), &bmp1); //get the first bitmap's data
GetObject(hbmp2, sizeof(BITMAP), &bmp2); //get the first bitmap's data

//if they differ in color depth or dimenstions they mismatch
if(bmp1.bmBitsPixel != bmp2.bmBitsPixel ||
	bmp1.bmHeight !=  bmp2.bmHeight || bmp1.bmWidth !=  bmp2.bmWidth)
	return 0;
else
{
        //get the bits of both bitmaps
	LPBYTE data1 = (LPBYTE)bmp1.bmBits,
		data2 = (LPBYTE)bmp2.bmBits;

        //the number of bytes per width
	DWORD bmWidth = bmp1.bmWidth * (bmp1.bmBitsPixel / 8);

	//somthing is wrong with bmWidthBytes member in Win9x and Win2k
        //so it's better to calculate the offset yourself
	DWORD bmWidthBytes = bmWidth + (4 - bmWidth % 4) * bool(bmWidth % 4);

        //number of bytes in data1 and data2
	DWORD bytesize = bmp1.bmHeight * bmWidthBytes;
        //compare the two bitmap's bits
	if(memcmp(data1, data2, bytesize) == 0)
		return 1;
}

GetDIBColorTable()