ScriptMistress:

Stretch Assistant

Simple and easy to use program that reminds you when it's time to stretch. If you spend most
of the day on the computer you probably aren’t doing enough to keep you from getting RSI.
The program reminds you to stretch you arms and legs so you don't have to get the pain of RSI.

Features
Very simple to use
Takes up little space on your desktop, can sit in the system tray
Start and stop the timer as needed.
Change the display message to whatever you like
Change the time to suit you, 45 minutes is recommended though

Stretch Assistant for $19.95