On Top window won't shutdown

I have developed a simple clock/calendar program where the clock is set "always on top" with a SetWindowPos API call, and the calendar is called up when desired with a mouse click on the clock. Problem: when the clock is active, it will prevent a system shutdown, i.e., "Start", "Shut Down", etc. will start the process, hang, and go back to the operating desktop; albeit with some programs (speaker control, etc) deactivated. Question: how can I make the program respond to a normal system shutdown? - other than manually closing the offending program first.