If you're having trouble getting movies to work correctly in PowerPoint, see Solving MultiMedia problems here and particularly see Austin Meyers' Multimedia Tutorial on this FAQ.
Make sure your media Plays For Certain describes Plays For Certain, an inexpensive commercial add-in that can solve most if not all of your "movie in PowerPoint" problems.
You'll find another detailed explanation of codecs, formats and PowerPoint's video handling quirks on Indezine's PowerPoint Video page
If running a movie seems to kill the mouse during a presentation (ie, the presentation won't advance to the next slide afterwards) and you're running Windows 2000, see MSKB article Q295810
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