WMP9 won't play MPEG1 files

I am unable to play an MPEG1 video file in Windows Media
Player 9 (9.00.00.3008). I get the following error:

"Windows Media Player cannot play the file. The file is
either corrupt or the Player does not support the format
you are trying to play."

The same file plays fine in RealOne player and QuickTime,
but not in WMP9. I need it to play in PowerPoint XP,
however, and apparently PPT uses WMP to insert and play
video files (I realize I can make a button that opens the
media file in an external app, but I want the file to play
inline in the presentation). MPEG1 is a supported file
type, so why can't I play it?