A client could not open *.pps files from an email. They would have to save it to their desktop, then open up the Powerpoint Viewer and navigate to the file.
I found a few sites that provided instructions on how to edit the registry to fix this – they did not seem to help them.
What did help was this:
Uninstall the existing version of the PowerPoint Viewer.
Then install the first PP View 2007
Then, install the latest upgrade – Powerpoint Viewer 2007 Service Pack 1 (SP1)
Then, go into your Control Panel and File Associations and choose *.pps and make sure that you point the file to the PowerPoint Viewer 2007 (SP1) and not the older program.
This seemed to work for them.