virtualdub has it's own internal Motion JPEG CODEC, which might allow you to convert it.
Morgan Multimedia have a Motion JPEG CODEC too, which is free for 60days.
Motion JPEG is kinda old now though. It basically compressed each frame as a JPEG file and then stored the JPEGs together in a file. Some of the old hardware capture systems would use hardware MJPEG to limit the amount of data that had to flow from the card to a more reasonable rate.