in my file it is also "XviD MPEG-4 Video Decoder".
in file wich come from satellite resolution is 720x576 and aspect ratio is 3:4,
but when i compress it in xvid aspect ratio becomes to 5:4.
here is screenshots:
http://www.saunalahti.fi/isos/5.4.gif
http://www.saunalahti.fi/isos/4.3.gif
mpeg4 modifier does not change it.