I think another thing you will need to do is when the time is up, you dont want it to flash like crazy saying "TIME IS UP!"
You probably want it to change the display to some text like "The presentation is about to begin."
Even better would be to let user set the text to display at end of timer.
More nice things would be:
1) remember the last settings (including text to display during countdown as traffic_doc requested), and text to display at end, and time, etc.
2) be nice if you could configure font size.
3) i think it would be good to have it exit also if the user presses the escape key. maybe it should exit if user presses ANY key or ANY mouse button.
4) probably better for it to completely exit instead of show its window again after you double click, as you will want to begin your presentation right away not have users see they timer program on screen and watch user go exit it manually before beginning the talk.