Office 2010 pptx file being d/l-ed as zip file

I have OS XP(3), IE8, Office 2010.  While this may appear to be a MS Office issue, please read on.

I am trying to d/l a pptx file from our organizational website (that I use regularly). The file is shown as a pptx file (as displayed by the properties feature), but when I click to d/l it, it shows up as a zip file to be opened and the computer tries to unzip it, but doesn't do a very good job in doing so.

I have tried to d/l other pptx files from other internet sites and they work (d/l) OK.

I have the same OS on another laptop and it d/ls this particular file from this particular site just fine.

I can change (rename) the extension on this file (from zip to pptx) and it works fine

I have tried everything reccomended on various PPT forums and it hasn't worked.

The file association is Microsoft PowerPoint.

The file d/ls perfectly with Firefox.

What is it about the configuration of XP/IE8 that lets this happen?