Easy. One API call. This function wraps that API call for convenience, allowing you to make the window topmost, or remove the topmost attribute with a single call. Remember that topmost windows occupy their own z-order : so if you make A topmost, then B, window B will appear on top of window A.

BOOL MakeWindowTopMost (HWND hWnd, BOOL bTopMost)
{
   if (hWnd)
   {
      return (SetWindowPos (hWnd,
                            (HWND)(bTopMost?HWND_TOPMOST:HWND_NOTOPMOST),
                            0,0,0,0,
                            SWP_NOMOVE | SWP_NOSIZE)) ;
   }
   else
      return FALSE ;
}