Cannot open pps files from Windows Mail

In order to view pps files, I downloaded and
installed the free pp viewer from Microsoft.
I checked the pps file association to be sure
they are associated with PowerPoint Viewer.
However, I cannot open the pps attachments
in Windows Mail. Rather, I have to save the
attachments to my hard drive and then open
them from My Documents.

What do I need to do to be able to open them
directly from within Windows Mail?

Thanks,
Mack