Reflexión is a small utility that will help you with that because it allows you to organize the periods of rest each certain time.
Enter the hour in the morning when you start to use the computer and Reflexión will calculate when you have to take the breaks.
Additionally, it's possible to configure a sound that will play at the time of the break. The program has one by default (a bell), but it can be replaced by any file in MP3 or WAV format.
If you don't want to use the sound alert, you can always turn to the typical text balloon that will be shown at the time of the break.