static CWnd* WindowFromPoint( POINT point );
point
Specifies a CPoint or POINT data structure that defines the point to be checked.
Identifies the window that contains the given point; point must specify the screen coordinates of a point on the screen.
A pointer to the window object in which the point lies. It is NULL if no window exists at the given point. The returned pointer may be temporary.
::WindowFromPoint, CWnd::ChildWindowFromPoint