Exclua o conteúdo do seu /etc/apt/sources/list
com:
#############################################################
################### OFFICIAL UBUNTU REPOS ###################
#############################################################
###### Ubuntu Main Repos
deb http://....archive.ubuntu.com/ubuntu/ quantal main restricted universe multiverse
deb-src http://....archive.ubuntu.com/ubuntu/ quantal main restricted universe multiverse
###### Ubuntu Update Repos
deb http://....archive.ubuntu.com/ubuntu/ quantal-security main restricted universe multiverse
deb http://....archive.ubuntu.com/ubuntu/ quantal-updates main restricted universe multiverse
deb http://....archive.ubuntu.com/ubuntu/ quantal-proposed main restricted universe multiverse
deb http://....archive.ubuntu.com/ubuntu/ quantal-backports main restricted universe multiverse
deb-src http://....archive.ubuntu.com/ubuntu/ quantal-security main restricted universe multiverse
deb-src http://....archive.ubuntu.com/ubuntu/ quantal-updates main restricted universe multiverse
deb-src http://....archive.ubuntu.com/ubuntu/ quantal-proposed main restricted universe multiverse
deb-src http://....archive.ubuntu.com/ubuntu/ quantal-backports main restricted universe multiverse
###### Ubuntu Partner Repo
deb http://archive.canonical.com/ubuntu quantal partner
deb-src http://archive.canonical.com/ubuntu quantal partner
###### Ubuntu Extras Repo
deb http://extras.ubuntu.com/ubuntu quantal main
deb-src http://extras.ubuntu.com/ubuntu quantal main
E veja se isso funciona.