In gnome there is a wonderful program which keeps track of user activity (movement bears work on the keyboard) and if the user is running continuously more than the set time, enables to lock the computer and the user is asked to walk for 5 minutes (also configurable). Unfortunately at work there gnome and can't see the name of the program, but something from the standard set of ubuntu.
Searched for something similar under windows XP. Preferably free, highly specialized (and not a huge program that eats 200 MB of RAM) and negligee.