You have a few choices here, but they range from poor to not-so-great.
Option 1 : Install OpenOffice and hope that it reads PowerPoint slides. My understanding is that there is some support but it varies, especially depending on how complex the slides are.
Option 2 : Install VirtualBox, then Windows something (xp,vista,7) and finally Office. This will give an exact and perfect viewing of you pps files, but you must have licensed products of above.
Option 3 : Download 'PowerPointViewer.exe' from MicroSoft and try to run it under Wine. This is a freely available product with licensing costs. As it doesn't actually modify the pps, but just display it, there's a good chance it might work.