Big Stretch Reminder is a freeware program which makes sure you take regular breaks from sitting in front of the computer screen, and so avoid RSI and other potential health problems. You set the time presets and the hassle level. I guess you also choose whether to ignore or obey too. Anyhoo, listen to your grandma, you need to take breaks. Otherwise your fingers will melt. Or something.
The program will sit happily in your system tray (where the little icons are in the bottom right hand corner of your screen.) until a predefined time period of your choice arrives at which point an alert will appear offering either RSI tips or a message of your choosing. Users also have the option of choosing an alert mode which can either be intrusive or not (depending on how strict you need to be), a micro-break can also be displayed on the screen if you wish.