Faz assim, clique em Aplicações > Acessórios > Editor de Texto, abra o arquivo /etc/apt/sources.list e cole o conteúdo dele aqui.
Olha ele ai, Cris!!!!!
# Sources.list para a família Ubuntu Feisty Fawn
deb cdrom:[Ubuntu 7.10 _Gutsy Gibbon_ - Release i386 (20071016.1)]/ gutsy main restricted
# See
http://help.ubuntu.com/community/UpgradeNotes for how to upgrade to
# newer versions of the distribution.
deb
http://archive.ubuntu.com/ubuntu/ feisty main restricted
## Major bug fix updates produced after the final release of the
## distribution.
deb
http://archive.ubuntu.com/ubuntu/ feisty-updates main restricted
## N.B. software from this repository is ENTIRELY UNSUPPORTED by the Ubuntu
## team, and may not be under a free licence. Please satisfy yourself as to
## your rights to use the software. Also, please note that software in
## universe WILL NOT receive any review or updates from the Ubuntu security
## team.
deb
http://archive.ubuntu.com/ubuntu/ feisty universe
## N.B. software from this repository is ENTIRELY UNSUPPORTED by the Ubuntu
## team, and may not be under a free licence. Please satisfy yourself as to
## your rights to use the software. Also, please note that software in
## multiverse WILL NOT receive any review or updates from the Ubuntu
## security team.
deb
http://archive.ubuntu.com/ubuntu/ feisty multiverse
## Uncomment the following two lines to add software from the 'backports'
## repository.
## N.B. software from this repository may not have been tested as
## extensively as that contained in the main release, although it includes
## newer versions of some applications which may provide useful features.
## Also, please note that software in backports WILL NOT receive any review
## or updates from the Ubuntu security team.
deb
http://archive.ubuntu.com/ubuntu/ feisty-backports main restricted universe multiverse
deb
http://archive.ubuntu.com/ubuntu/ feisty-security main restricted
deb
http://archive.ubuntu.com/ubuntu/ feisty-security universe
deb
http://archive.ubuntu.com/ubuntu/ feisty-security multiverse
########### Outros ##################
#############################################################
# Todas linhas com "#" no início não serão interpretadas pelo Linux
# Ativar ou descomentar = retirar o # do início da linha
# Desativar ou comentar = colocar # no início da linha
# Após modificações, salvar o sources.list e depois no terminal