Powerpoint problem here..?

I downloaded some powerpoint presentations to use it as a reviewer for our computer systems final exam next week, but the problem is those powerpoint presntations are all the .ppt format.. I tried opening them up in my computer, some of them would open but the others would not.. what should I do?? I really need them , I don't want to fail next week... by the way, I am using windows vista and microsoft office powerpoint 2007 is installed.. thanks for your help!