Here is a decent free downloadable countdown timer that will run in full screen mode on your computer.
mouse over and use right menu to control clock ( also ctlr-f for full screen mode)
You can easily customize the view by
1) opening the main window
2) select timer in the list o the left
3) select "edit" timer settings
4) select the "font" check box instead of the skin (font =tahoma)
5) change the start countdown time to 2 min 30 secs