You can download the PowerPoint viewer from Microsoft free of charge and this will allow you to view .PPS files. Please note that you will not be able to edit or modify these files in any way - for that you will need the full version of PowerPoint.
This link shoudl give you what you need:
http://office.microsoft.com/downloads/Default.aspxHope this helps.
Michael Gurner
Brainbench MVP for MS Excel
http://www.brainbench.com