Running a Mac, have you tried using Keynote that comes with either iWork'08 or '06 and import the PPT file. This has worked for me recently as I'm still running 2004 and one of my clients has recently upgraded to 2008 and experienced a similar issue. Not guaranteed to work for all PPT documents as the content contained within the document varies, but worth a try.
How to: Simply within Keynote under File/Open, open your PPT file, then choose File/Export, select PPT and do a Save As from Keynote. Then open the new exported file in PPT08 and see if this helps.
If you don't have iWork'08, download it and use it in demo/tryout mode.
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