I've foudn a straightforward way to resolve this problem.
0. Work with a copy of your presentation.
1. Open the troublesome presentation in MacPPT.
2. Select an image that does not show correctly when opened under WinPPT.
3. Select "Picture Object" from the Edit Menu.
4. A dialog box will appear: "This is an imported picture, not a group.
Convert it
to a Microsoft Office drawing?" Your choices will be "No" and "Yes".
5. Click "Yes".
6. Repeat this for each offending graphic. Do this only once per graphic.
7. Save the presentation, copy to WinPPT and view the results.
Worked well for me, but I tried it only on one presentation. I've been
working cross-platform between Mac and Windows for years but have not run
into this before. Probably my work habits versus those of the creator of the
offending presentation.
Cheers!
Ott