Mplay32.exe plays video but PPT 2007 doesn't

[Ronsky]

I received some video files that Gspot says use WMV3 codec. They play
fine in Media Player mplay32.exe but won't play at all when inserted
into PowerPoint 2007 using the Media Insert tool. A white box is
displayed.
I've tried running with and without Hardware Acceleration with the
same result.

Does anyone have suggestions of other possible causes?

I have transcoded the files with WMV2 codec and they run fine.
Why not with WMV3 and why do they play in Media Player?

Thanks!