Reinstall the camera's driver software. This refers to an CoDec and since there is no standard MotionJPEG CoDec in Windows, it must have come from your cam software or possible a third-party CoDec pack.
Many thanks for your response
However there is no camera software that I am aware of. You simply connect the USB cable to the camera and copy the images to the PC. I can then double click on them in WIndows Explorer and they play fine but not in AE
However since posting this my son has found that if he does create movie in AE (something to do with creating a proxy?) it then plays OK
DOes this help explain what is going on?