It is possible that something has hijacked the file association for PPS.
Open Windows Explorer and go to Tools > Folder Options > File Types and
locate PPS. Make sure that it says "Opens with: Microsoft PowerPoint for
Windows." If it says something else, change it to PowerPoint . Click on
Advanced and under the Actions, locate Show and click on Set Default. Show
should now be highlighted. OK your way out and try clicking on a PPS file.