Unless you're one of those blue aliens from Avatar, you're susceptible to repetitive stress injury on account of your time in front of your computer. (The Nav'i, you might remember, have super-strong bones, which would, I think, protect them from this sort of thing. Sheesh, I'm such a geek.)
You can wait until you feel a sharp, debilitating pain in your wrist, or proactively work to preserve your body right now, before you damage anything. If you choose the latter, then I've got a program for your PC that will help you stay healthy.
Workrave is a free program that monitors how long you're working at the PC, lets you know when it's time to take a break. Indeed, the program keeps you informed about when your next break is coming up and even lets you know when you should quite working for the day. When it's time to push away from the keyboard, Workrave coaches you through a handful of easy exercises you can do right at your desk.
And for some more exercises you can do right at your desk, be sure to check out a video that demonstrates exercises that prevent tendonitis and carpal tunnel.