It's not necessary (and sometimes not advisable) to mention things like "I like this job" & "I really want this job". The fact that you want the job is obvious - you're applying for it. Rule #1 of job applications is: never apply for a job you don't want. It'll shine through.
Keep questions objective, informed and to the point. Try to show through the questions that you have the required insight into whatever knowledge the job requires.
You should also think about what you would answer if they ask "Why should we hire you". If they don't ask it, try and get some of the key point of why they should hire you through during the questions and/or smalltalk. If you can't think of any reason they would hire you, don't apply for the job.
When I applied for part-time jobs as a student the interviews where never that rigorous though. Most of them were 30-45 minute talks with maybe a little quiz and brainteaser thrown in. But when I interviewed for proper engineering jobs ... phew.

Why would you send a letter? Thank him/her/them there and then. To follow up on your application post-interview, well you presumably have a phone. Use it.

Dude, what is your job? Hahah.

IT tech support at a high school.

+1
Having a pet project of any description - whether it's a home network running Active Directory or building a database-bound website for a clan - demonstrates an interest in IT and personal motivation.
Having been on an interview panel for support staff, here are a couple of questions which have caught applicants off guard:
* "Give an example where you've had a conflict with another person in a personal or office environment, and how you resolved that conflict"
Questions like this are complete balls, but are possible if the uni has a HR department. The key to answering these is to be relaxed, ask for 30 seconds to think, and then answer honestly. They would rather you say "I haven't had that experience", and perhaps offer a hypothetical solution, than to hear you dredge up some unrelated example of a guy you didn't like in high school.
* What are your career goals / where do you see yourself in 5 years?
Any employer wants to know that you will see value in working for them. If your ambition is to be a programmer, support is a day job but at least it's in the right industry. If your long-term interest is in systems administration, then support is part of a possible career path. If your goal is to be a chef or stay-at-home dad, that's not a good sign (unless you promise to bake cakes for them every Friday).
Be honest and ask questions. If you have a real interest in a job like systems administrator at the uni in a few years, say so and ask if this job would be a good stepping stone.
... and when the interview is done, it's fair to ask when you can expect to hear back from them. If you don't hear back from them, call them to follow up. Sometimes employers are lazy and it helps show that you are motivated, and sometimes they're just inconsiderate and never send out rejection letters. Either way, it's best to know.
+1 to this (and everything else stonebro has said, though I disagree with the decided-in-the-first-5-seconds statement).
Remember that if you're successful in the interview, you'll be offered a job... and if you accept, you'll be working there.
The "do you have any questions?" question is your chance to discover that your job sucks before it's too late.
If you're a social person, maybe the size of the support team is important to you. Or maybe you're wondering if you'll be expected to drive between different campuses. If you feel the interview is going well, you may even want to ask if you could be introduced to the position's supervisor. He (or she) is probably in the interview panel, but if not, then it's important to at least say hello.

Bring a towel ...
Also, get someone else to lie in front of the bulldozer for you ......wait wrong answer.
But, bring a towel.

You're still alive? Damnit I owe Weclock $20

Reports of my demise have been greatly exaggerated ...also I want in on that $20. :D

I feel like Mr. Kotter for some odd reason....:D.
and Thank you!

Ha! Keep dreaming, I'm not even paying weclock

I'd tell you to at least buy him a GOG but he doesnt like anything here, haha.

A couple of suggestions:
1. When interviewing for a tech position, never ever pretend to know things that you don't know. You will probably get quizzed on them by the guy who really does know.
2. Be honest. Really. But don't bring up stupid things like prior drug use or naked women. Don't be negative though.
3. Entry level tech positions are often decided on "cultural fit" and "ability to learn". Make positive comments about the work environment (unless it really does completely suck). If they ask you "do you think you'd enjoy working here?" The obvious answer is "yes, and here's why." (Unless that's a lie.) If you get a question you don't know, admit you don't know it but express a willingness to learn. The perfect candidate for a tech position "knows 80%, can learn the other 20%". If you know 100%, it can often cost you the position because you'll come across as a short timer (somebody who is going to quit when they get something more interesting.)
4. Be friendly but not a suck-up. Show the ability to think for yourself.
5. Turn OFF your cell phone, or don't even take it in the building with you. Using a gadget during an interview will cost you a job even if you're obviously qualified for it.
(I laughed out loud at the khaki pants comment above. I'd recommend however just not peeing your pants.)
That's all I can think of at the moment....

I don't agree with the thank you letter. In my personal opinion it's a suck-up move and won't increase the chances of getting a job. In most cases I've either had a job offer or a "no" long before snail mail would have arrived at the office.
Maybe a thank you buried in an email with some other relevance would be OK.
Just my opinion though.

I probably wouldn't do that. But as I mentioned in my post above, you will probably be judged on "cultural fit", which means "how well do you fit in with the people and work environment?" So it doesn't hurt to make specific comments about why you'd enjoy being there, and why you'd be a benefit to the team.

Second barleyguys thank you letter notion.
All good advice. :)

I disagree on a thank you letter being a suck up. It's often viewed as a courtesy.
Depending on the culture where you're interviewing, your prospective employer may be looking for that. It's common where I'm from to send a followup letter (or at the very least an email, as others have suggested) and even to give a followup phone call a week later to make sure they received the letter.
That if nothing else will give you an opportunity to ask if they had any questions you could answer, or you could ask them.

Hmm, I'm not sure on the follow up letter, I've never heard of people doing that before.
Adding to my qualifications, I passed my network engineering exam today which is the last test I'll ever do, my remaining 2 subjects next semester are pure assignment marking, I'm good at those subjects and both the lecturers are great people who like me and the only way I'll fail those is if I don't try so I've pretty well got the degree. I can bring that part up as a positive