The producers of Rokslideshow have developed a powerful free module to display a powerpoint on your Joomla website.
Update: Time moves on and this module appears to be obsolete - please check out this module for slideshows on your joomla website.
The following example comes from a site we developed for a technical assistance program in Afghanistan.
We created first a powerpoint presentation and saved all slides as an image. The images are shown by the Rokslideshow module, although it does look like a powerpoint presentation. You can do the same with Adobe Presenter, but it will cost you $500 or so. Our solution is for free!
Rokslideshow can be found in the JED.