This always happens to me, as well, when I am projecting the ppt. For me, it
has to do with the video output on my computer. There is a way to toggle (3
way toggle) between video output modes (on my laptop it is Fn+F4) so that all
the output goes to the projected screen OR to your computer-screen OR to
both. The default is "both" so that you can see the movie on your computer
screen and also on the projector screen at the same time but MY computer will
not do this: it will only project the movie if it is toggled to projected
screen only. (Actually, it will project some avi's to both but most of the
movies I need to project are other formats.) You can do a general search
under "video output" + powerpoint.
Myst