Whilst I haven't used Windows for a long time, could this be something to do with the email program?
Does the default application for *.pps need setting up for Outlook or Outlook Express?
Just a thought.
Obviously all thoughts are better than none at all, and who knows just one thought might just shed some light onto the problem.
But there is no need for any extra settings to enable attachments to work, all that is required is that the appropriate program is installed, which in this case is PowerPoint.
Not that it probably makes any difference either probably just down to being a different version of MS Office but my PowerPoint files show with the extension of .ppt.
The attachment will not play directly from the email program but will play when moved to the desktop, very strange to say the least.
'Which email program are we talking about?'