IT Departments rarely get any respect, we're firmly placed on the 'Rodney Dangerfield' rung of the office social ladder.
People seem to enjoy blaming IT for anything that goes wrong in their office. Microwave dies? IT should be fixing it (trust me, I've been ASKED to do that job... no joke).
I was in your situation a few years ago. I've been doing IT for 7 years now, at first I was a happy go-lucky fellow with patients to take on the world. But... that wears thin when your co-workers treat you like another machine to do their complex tasks (or some of them do).
Everyone expects technological miracle, and they expect them to cost their department $50. My advice to you is to get a good sense of humor and a thick skin, you'll need both. Make people laugh and you disarm them a bit. I always try to have some sort've amusing 1 liner to give my bosses when something is nearly an impossible task, and they get the picture.
Another important thing that I've seen mentioned here is documentation. You need to document everything and anything that goes on while you're at work. Keep a log of your daily work activities, the projects you've got going, any problems that occur during the day (and the steps you took to resolve them).
The most important thing to document is altercations with people that you have, and they will happen. Remember first off if someone is being beligerant towards you, keep your calm. Yelling back at them will only escalate things and get you involved in any of the negative effects. Also any witnesses available get them to write out what they saw from their perspective and sign off / date it ASAP. This is annoying, but it's saved my hide time and time again. If you've got 3 people agreeing with your side of the story, the other individual generally doesn't garner any sympathy.
Lastly if all else fails? Violent video games... just don't start picturing peoples heads on your enemies and you're golden.
Hope that helps
-K