Question in interviews are always in "essay" format. In other words, they want you to discuss the answer and give them an overview of the answer. This can be a challenge for many of us that don't really speak in "essay" format. However, this is a hump that has to be conquered if you want to improve your interviewing skills.
Here are a few questions and answers that may help:
Question: Tell me about yourself?
Why: This is one of the most frequently asked questions you will ever get in a help desk interview. The interviewer makes this question vague so that he can grade you on where you take the question.
Advice: Answer the question in a way that prompts questions and be a little vague yourself so the interviewer can steer you a bit. You don't want to bore him to death by talking about your Disney vacation while you were in grade school, but you want to have "ammunition" for as long as he can stand listening to you drone on. Also, try to talk about your technical skills, experience, and your general interest in IT. Don't be afraid to show a little personality either, IT managers would rather hire someone they like, than someone that knows everything about IT.
Example: I grew up in the suburbs of Chicago and have pretty much stayed there my entire life. During high school I worked at X, fixing up old computer for resale. Have you ever been to X? Well, they are a small company near the city that buys old equipment from companies in the area, fixes them up, and sells the refurbished equipment at auction. I had to leave the company when I went away to college, but it was a regretful decision. I really enjoyed working with the people over there, and liked the fast pace environment. During college I worked in the IT department as an intern and spent most of the time removing malware and other similar issues, setting up network printers, and the usual day-to-day help desk type work.
Summary: During the entire answer you were talking about your skills indirectly, you showed that you like working with people, and you went out of your way to let the interviewer know that you like a fast paced environment (all help desks are). You asked questions making the entire session interactive and the single question could have turned this into more of a conversation (two-way), than a dissertation.
Question: Why Should I Hire You?
Why: This question is meant to make you a little uncomfortable. The interviewer wants to see if you can talk him into hiring you.
Advice: Be a little humble and talk more about how you can grow in the position and less about how you already know everything.
Example: I will hit the ground running and be sure to learn everything I can as quickly as I can. Your technology environment is very complex and I look forward to jumping in with both feet and learning all I can from day one. This is a big opportunity for me and I would do everything possible to make sure I succeed in the position.
Summary: This quick answer speaks volumes about you and what your goals are. You are telling the interviewer that you want the job. This is the most important message to convey. In addition, you are suggesting that you will be an asset from the first day, and your value to the company will grow exponentially every day after your first day. Finally, you are telling him that this isn't just a job for you. This is a "big opportunity", this is a great closer.
Question: What is your biggest weakness?
Why: This is another question that is designed to make you a little uncomfortable. Describing your weaknesses to a perfect stranger is not easy to do.
Advice: I've given hundreds of job interviews and I can't say with any certainty that anyone ever answered this question perfectly. Try to talk about a weakness you have that may be construed as strength.
Example: I tend to become obsessed with difficult to solve problems. I know it often makes more sense to escalate an issue to someone that is more familiar than I am, but this process may elude me sometimes.
Summary: You are telling your interviewer that you want to fix everything you see, but are willing to succumb and escalate the problem if that is part of their help desk processes.