As long as you know where CTRL, ALT and Delete are, you'll be fine.
Might be worth asking what platforms they have (Mac, Windows, Unix, etc), if the opportunity arises to show you're interested in learning about their infrastructure. Say if you have experience in the bits you know, keep schtum about the bits you don't. If you're not well versed in Mac OS X, for example, don't mention it unless you're asked. If you are asked about it, say that you've not used one since xyz, but you're sure it'll be easy to pick up. Shows confidence in your ability to learn something new, and that you're aware of new developments, even though you're not as up to date as you could be.
Is is first or second line support? I sadly do both. I AM the helpdesk.
Puts a real gripper on when you're concentrating on something important!
Just remember never take the user's word on what they say the problem is - always investigate for yourself.
Always ask the stupid questions first - "Is Caps Lock on?" solves 99% of password problems.
Always get them to check while you're on the phone - a problem might have occured half an hour ago but they've been to busy/bone-idle to report it and by now it's not a problem any more.
You'll probably only be asked to cover what's on your CV and past experience, maybe go through some hypothetical situations. As each business and it's IT infrastructure is different, you'll only really get quizzed on 'general knowledge' and generally accepted sound practice, ie backup before cockup.
Never say "I don't know". It's always a bad answer, whether in the interview or on the job. "I'll find out and get back to you" makes you sound less ignorant - won't work in the interview though
so ask to clarify what they're asking if you don't understand.
Don't be intimidated by the questioning, and don't scratch your ear or your nose - its a sign you don't believe what you're hearing or saying. Don't blabber - keep your answers to the point, and expand 'generally' - "This one time, at band camp..." won't do you any favours.
Make sure you learn about the company before you go in - remember that episode of the Apprentice where someone was asked "Do you know what Amstrad do?" and they said "No"?