When you're going for an interview there's no hard and fast rule about whether you should present your portfolio on a laptop. Just remember that design for the screen always looks best on a screen, and design for print looks best on paper.
But if you do take a laptop, chances it will be a MacBook, MacBook Pro or a PowerBook (which you either own, or you've begged or borrowed). Here's a tip: close the lid on one of these and it's asleep; open the lid and it's awake. You don't need to start it up during the middle of an interview - that's just a waste of a precious five minutes (and won't do you any favours). And don't take the power lead along either - make sure the battery's fully charged when you arrive (which means shutting down all the programs except the one needed for the presentation).
And, while you're about it, make sure the desktop is tidy and that your presentation can be activated with one click. You won't get the job on the basis of demonstrating your skills in searching the hard disk.