I think that Maver1ck666 may be on about the fact that MPEG's come in four categories -
MPEG-1 - which is optomised for CD-ROM
MPEG-2 - for broadcast quality video
MPEG-3 - which was merged into MPEG-2
MPEG-4 - optomized for low bandwidth video telephony
There is a piece of software here that will do a conversion between the various compressions (I think):
http://www.heroshare.com/products/VideoConvert.php
It's not free ($29.95) but offers a free trial. I've never used it so I'm not sure if it's any good.