I used Google docs (see google) and I can upload, edit, download a presentation all with worry or cost. If you want you can try Open Office on the PC and NeoOffice on the Apple.
For one of my classes at school, I have to create a PowerPoint presentation. The presentations are mostly done at school, and what we don't finish in time is done at home, which is what I have to do. However, the computers at school all run Windows XP, while my computer at home is on Vista. Also, the version of PowerPoint we use at school is 2003, while at home I use the newest version (2007, I think). Since all I'll be doing is saving the presentation from my email at school and viewing it through the earlier version, will there be any compatibility issues between the file format and operating systems/software versions?