OutlookExpress / File Association

I have downloaded the PowerPoint viewer from MS - it installed well. I have an email with a PPS (PowerPoint Slideshow) attachment. When I attempt to open it from within OE, a complaint is raised that a file association doesn't exist. However, when I save the file and double click on it, the viewer opens and displays the file as expected - without any complaints.

Why the discrepancy? Why won't OE recognize the association that exists?

Thanks.