Failed to fetch http://medibuntu.sos-sts.com/repo/dists/feisty/free/binary-i386/Packages.gz 404 Not Found
Failed to fetch http://medibuntu.sos-sts.com/repo/dists/feisty/non-free/binary-i386/Packages.gz 404 Not Found
Failed to fetch http://medibuntu.sos-sts.com/repo/dists/feisty/free/source/Sources.gz 404 Not Found
Failed to fetch http://medibuntu.sos-sts.com/repo/dists/feisty/non-free/source/Sources.gz 404 Not Found

update the lines about medibuntu in source list (/etc/apt/sources.list) with :

deb http://packages.medibuntu.org/ feisty free non-free
deb-src http://fr.packages.medibuntu.org/ gutsy free non-free