It's a good idea to take frequent breaks from the computer during your work day in order to help prevent repetitive stress injuries. Most sources I've read have recommended taking a break at least every half hour for 2-3 minutes, with some recommending a break every 15 minutes.
There are a number of break reminder programs out there, some free, some that cost $20-30 (most of these have a free trial). I'm on the hunt for a good one right now. I was just doing the Oh-I-know-when-it's-been-20-minutes thing, but it's easy to let an hour pass without realizing it.
I'm going to try a few and I'll let you know which I like best.
I've been using Albion StopNow for about two days, and so far it's the one to beat. It doesn't have any annoying popups that knock you away from your current window, and it recognizes when I'm using Drgon NaturallySpeaking and counts that as time. It resides in the system tray and turns red and beeps (annoyingly if you ignore it--but that's the point) when it's time to take a break. You set the break lengths and times during the fst and easy setup. The only downside is that if you don't generally work with your speakers on, it'd be somewhat easy to miss the warning and just work through. Albion is free to try for 10 days, and then you have to pay $25 to keep it.
I downloaded Break Reminder which is a free program, but the setup was 11 steps, and I was tired of messing with it by step 7. There's too much junk about micropauses and what should be counted as work time and break time and yadda yadda. After Albion, it was a pain so I said forget it early on.
I don't mind paying the money for something I like, so Albion remains the one I'm using for now, but I'm going to try a few more programs before the 10 days is up and see if there's something as good out there that is free. If I find something better, I'll post it.