To answer a few questions...
Why I have so little to do: They use a needly complex system and the previous temps have struggled to grasp it. I learned it fairly quickly. Combine that with the fact that I'm a pretty efficient gal, I get done in 2 hours what took previous temps 4-5 hours. Also I'm expected to handle the switchboard, so even if I don't have any active work I have to be there to answer the phones.
Helping other workers: I am tied to my desk, again because of the phones. So I am not free to hang out with coworkers to learn more or chat with them.
Improving skills: a good idea but I'm not sure where to start. I work in Excel all day and I've never run into a situation that's out of my depth there. I don't use powerpoint in my field.
My temp agency: They already know that it's not working out and are trying to find another assignment for me. This is a problem client who's been through 4 other temps already for this position. I haven't discussed the boredom issue with them because I wanted to focus on the most important issue and not make it seem like I'm whining about every little thing. Plus, since I/we are actively looking for other work for me, I'm afraid it might come across as whiny if I were to talk to them about this issue.
All in all, I really appreciate all the advice offered. I did start a personal writing project today and that definitely helped the time pass.