Simplemente pon esto en tu archivo de configuracion mirror.list
####QUANTAL32bits#####
deb-i386
http://archive.ubuntu.com/ubuntu quantal main restricted
deb-i386
http://archive.ubuntu.com/ubuntu quantal-security main restricted
deb-i386
http://archive.ubuntu.com/ubuntu quantal-updates main restricted
#deb
http://archive.ubuntu.com/ubuntu quantal-proposed main restricted
#deb
http://archive.ubuntu.com/ubuntu quantal-backports main restricted
#########QUANTAL64bits###########
deb-amd64
http://archive.ubuntu.com/ubuntu quantal main restricted
deb-amd64
http://archive.ubuntu.com/ubuntu quantal-security main restricted
deb-amd64
http://archive.ubuntu.com/ubuntu quantal-updates main restricted
#deb-amd64
http://archive.ubuntu.com/ubuntu quantal-proposed main restricted
#deb-amd64
http://archive.ubuntu.com/ubuntu quantal-backports main restricted