MPG not playing in PowerPoint

this happened to me when i tried to change an mpeg to avi by renaming. it will work on occasion in wmp to divx but check to see if the media hasen't been altered. in other words, GSPOT the vid to see if it's truly a mpeg... that will always cause green screen....might not be it but you could always run it though vdub to see if the problem is consistent with all other media.
just a thought