If you're going to do it, do it well. Get a mac, too. I'm serious, the graphics are smoother and the fonts more legible in presentations, and that means fewer distractions. Don't use cheesy colors, bright annoying combinations, complicated transitions, or anything else that screams "kid with a new toy". K.I.S.S.
And, don't use Powerpoint to make up for good speaking/teaching skills...it won't.