Drag the attachment out of the e-mail window onto your desktop. Then, right-click on the .PPS file that's now on your desktop. From the pop-up menu, choose "Open with...". Select "PowerPoint" or "PowerPoint Viewer" from the list of programs, make sure the checkbox "Always use the selected program to open this kind of file" is checked, and click OK. See video for example (sorry, can't attach a video file to Expers Exchange):
http://8.20.78.80/PPSFiles.wmv