My most recent
guess is it has something to do with the attachment extension.
Check the properties in the future before you open. If the extension is
attachment.
PPS this would start a Microsoft Power Point Slide Show.
You can set what application open what extensions via
start > Control Panel > Folder Options > File Types
.PPS is default opened with Power Point Show.
So if the sender is sending you a
.PPS attachment then that's why the slide show is opening.
The next question is why some of your local files are being mixed into the show?
I will look further.