I can't tell you how many interviews I've conducted for Help Desk I positions... primarily because I simply don't remember, but I've hired quite a few folks. Here's the feedback I have for you.
First off, be confident.
You'll hear that often, and often times people mean 'act like you deserve the job.' On the contrary. Be confident in what you know AND what you don't. If you don't know the answer, don't be afraid to say so right away, make a guess, and offer to get back to them with the right answer.
Second: Be likeable
What I mean is, always try to connect with people on an interpersonal level. This can be done by the proper use of mannerisms (sir/ma'am/please/thank you) but can also be done through deliberate and equal eye contact, smiling, and open body language. e.g. Don't lean back in your chair, or cross your arms. Lean forward, hands folded on your lap or on the table for quick access to gestures.
Third: Do not 'word vomit'
The words 'um' 'uhh' and the like should never come out of your mouth during an interview. Well, how do you answer difficult questions then? Buy some time! This is very easy to do without even seeming like you're doing it.
Scenario:
Interviewer: Describe a time when your best friend did something and you had a moral obligation to act. What was the situation? How was it resolved?
Now, you can start talking and sound like an idiot with all the uhmms and uhhs, or you can carefully construct a reply by buying time. How? Paraphrase the question back to the interview to confirm understanding, wait for them to answer, and then state that's a very good questions. This gives you a good 30-45 seconds to prepare a kick ass response.
You: "To be clear, you're asking about a situation where a friend of mine did something that created a moral obligation to act on my part, and how it turned out, correct?"
Interviewer: "Correct"
You: "Great question, (3 second pause), [now your prepared scenario]"
Finally (I've got more advice, just dwindling time): Know the job
When I worked on the help desk, and you can ask anyone here who does, most of your 'resolution finding' is done through...(drumroll) Google! Why not bust out some Google ninja skills in the interview? I did this in my interview when I was first hired. They asked me what RAID 10 was. I wasn't familiar with the specific number designations. So, I pulled out my iPhone and told Siri to Google Raid 10 and gave them an answer right away. I understood the concept of RAID, so I would definitely advise against doing this for every answer you don't know and especially concepts you're unfamiliar with, because you may end up sounding like an ID10T, but it can show that you know how to find information, even though we all know that's what you'll be doing later.
Best of Luck!