Many authority's recommend a small pause (1 à 2 minutes) per hour of computer work.

Reflexion is a tinny application that prompts you kindly when it is time for a pause. It runs in the windows notification area (also known as tray area). You can set the start- and stop time and the number of pauses you like. A list is dynamically updated and showing all the pause occurences. Optionaly one can play a sound (wav), mp3 or wma file to prelude the pause.