VOID PASCAL ScreenToClient(HWND hWnd, LPPOINT pPt)

  This function translates a point's coordinates from absolute screen
coordinates to coordinates which represent an offset from a window's
client area.

Parameters
  hWnd is a handle to a window whose client rectangle will be used in
   the transformation.
  pPt is a pointer to a POINT structure whose coordinates we will transform.

Returns
  Nothing.