Eddie said: "If you receive any file as an e-mail attachment, you 'should' first save it to a folder (or Desktop) then open it. Not every type of file will open properly when you try to open it while it's still attached to an e-mail."
It has also been my experience that .pps (and other) attachments are best downloaded and viewed that way, anyway, so I would not worry about not being able to view them in the email window or messing with the registry, if that bothers you.