Files that end in ".pptx" and ".ppt" are Microsoft PowerPoint files (unless they are improperly named, of course).
The ".pptx" file extension indicates that the file ware saved in Microsoft's XML-based file format, whereas the the ".ppt" format indicates that it was saved in earlier versions of Microsoft's file format.
If I understand correctly, newer versions of Powerpoint automatically use the pptx format. Unfortunately, older, but still recent, versions of Powerpoint are unable to open pptx files.
Thankfully OpenOffice, a free and open source office productivity tool, can open and save in both formats. Today I used "ooimpress" to do just that. The only problem I had was due to a missing font, so I simply made some adjustments and saved in the ppt format.
UPDATE: I just received confirmation that the Windows user who was unable to open the .pptx file was able to open the .ppt file that I converted with OpenOffice!