Download OpenOffice.org which contains, several free software programs, including OpenOffice Impress -- the presentation component.
This program is a wonderful resource to have, for several reasons. To begin with, if you are already familiar with PowerPoint, you will have absolutely no trouble at all navigating around in the program -- it is that similar. Also, any PowerPoint presentation you have already created (version 2003 and earlier) should open with no problems. (However, be aware that if you have added many special effects such as transitions and animations that are specific to PowerPoint, those may not work). You can also save the presentation in a PowerPoint format. All and all, a great resource for those who do not have PowerPoint available on their computers -- or even if you do.
Get Started with OpenOffice Impress
Beginner's Guide to OpenOffice Impress