Here's one you may need after reviewing all these other free programs....
It's called Nagger and is a serious attempt at trying to reduce the genuine health & safety problem with computer over-use (coincidentally mentioned in a radio news item today).
Ideal for any work-a-holics; I know how difficult it is to drag yourself away from the computer when you get engrossed :x
It basically reminds you to take a rest from the computer after a certain period. It's very easy to use; no bells and whistles but it does the job effectively.
The default settings are as recommended by Health & Safety (10 minute rest period every hour), but you can configure it how you like. You can override it if you really must (some deadlines just can't be ignored), but it makes you feel guilty of the dangers as you have to type in one of the over-use symptoms ;)
Screenshots, more details & download (free) are on the Freebies/Software section of www.philizound.co.uk
Hope you like it.