Well you're going to need software that can handle a .ppt file.
Your best solution would probably be to install OpenOffice.org - there's an app in that suite (Impress) that allows you view .ppt presentations. I don't know if there's any "lighter" software (e.g. for viewing .ppts, without any editing abilities), but you're probably going to need an office suite sooner or later so getting OOo seems a good solution to me.
I also think it's possible to view presentations via Google Docs (docs.google.com), but I've never tried it.
Last edited by Maligree; 02-12-2009 at 12:25 PM.