You're sure that it's ending in a .ppt extension? There really isn't
anything you can do other then take a good look at the presentation filename
and the file itself. The web browser goes off the file extension coupled
with the MIME type transmitted by the server. There's nothing in HTML you
can do to fix this.
Tomake sure that the file really ends in a .ppt extension, you'll need to
turn off the hide file extensions in the Windows Explorer Tools | Folder
Options | View menu hide extensions for know file types.
Also, check to see if another computer does it. That will at least tell you
if the associations between filetypes and applications is correct or if
there's a problem.