Hmm, then maybe I should have spelled out that MS Office 2003 and before could not naturally open pptx files without an additional package. The compatibility pack introduces the capability but does not stitch up the file itself, which remains as is.
I'm still using MS Office 2003 on Win 8.1 with the compatibility packs (despite MS implying you can't). I've found no reason to pay for a later version as it does all I need.
Always pop back and let us know the outcome - thanks