Are you just wanting to be able to view PowerPoint presentations? They should open in Keynote for you if you have iWork installed.
From the back of my iWork '09 package:
iWork makes it easy to exchange documents with anyone.
Open Microsoft Word, Excel, and PowerPoint files in iWork
Save your iWork documents as a Word, Excel, or PowerPoint file.
there is a free alternative for Microsoft Office:
OpenOffice (Oracle), or LibreOffice, or NeoOffice.
They are all practically the same. The only difference with the Microsoft Office package is that there is no Entourage (email/contacts). You have to set the different parts to save in the MS Office format (like "doc", "pps", "xls, etcetera).