You can use the GetTopWindow function to search all child windows of a parent window and return a handle to the child window that is highest in z-order. The GetNextWindow function retrieves a handle to the next or previous window in z-order.
GetTopWindow: http://msdn.microsoft.com/en-us/library/ms633514(VS.85).aspx
GetNextWindow: http://msdn.microsoft.com/en-us/library/ms633509(VS.85).aspx