Ergotimer

Provides relief from repetitive stress problems by monitoring the mouse, the keyboard and the time spent at the workstation. When the programmable number of mouse clicks, keystrokes, or minutes occur, a break screen pops up and counts down your break time.

A password enforcement option is also supported for ensuring that break time is not interrupted. Ambient Soothing Sound.

You can try Ergotimer (tm) without fear as it doesn't change any system configuration.