Vamos lá, abra sua sources.list com um editor de texto de sua preferência (vi, vim, nano, pico, gedit, pluma, etc...) e apague todo o conteúdo do arquivo, e cole o conteúdo abaixo.
#############################################################
################### OFFICIAL UBUNTU REPOS ###################
#############################################################
###### Ubuntu Main Repos
deb http://br.archive.ubuntu.com/ubuntu/ precise main restricted universe multiverse
deb-src http://br.archive.ubuntu.com/ubuntu/ precise main restricted universe multiverse
###### Ubuntu Update Repos
deb http://br.archive.ubuntu.com/ubuntu/ precise-security main restricted universe multiverse
deb http://br.archive.ubuntu.com/ubuntu/ precise-updates main restricted universe multiverse
deb http://br.archive.ubuntu.com/ubuntu/ precise-backports main restricted universe multiverse
deb-src http://br.archive.ubuntu.com/ubuntu/ precise-security main restricted universe multiverse
deb-src http://br.archive.ubuntu.com/ubuntu/ precise-updates main restricted universe multiverse
deb-src http://br.archive.ubuntu.com/ubuntu/ precise-backports main restricted universe multiverse
###### Ubuntu Partner Repo
deb http://archive.canonical.com/ubuntu precise partner
deb-src http://archive.canonical.com/ubuntu precise partner
###### Ubuntu Extras Repo
deb http://extras.ubuntu.com/ubuntu precise main
deb-src http://extras.ubuntu.com/ubuntu precise main
Depois, execute os seguintes comandos abaixo.
sudo rm -r /var/lib/apt/lists/* ; sudo apt-get autoclean ; sudo apt-get autoremove --purge ; sudo apt-get update ; sudo apt-get upgrade ; sudo apt-get dist-upgrade
Ola agregando, provavelmente a sua lista nao foi salva e tudo se manteve na mesma porque para abrir esse arquivo precisa ser root entao, agregando ao que o Arthur reportou;
sudo gedit /etc/apt/sources.list
Apague todo o conteudo e ai vc cola o que ele te passou;
#############################################################
################### OFFICIAL UBUNTU REPOS ###################
#############################################################
###### Ubuntu Main Repos
deb
http://br.archive.ubuntu.com/ubuntu/ precise main restricted universe multiverse
deb-src
http://br.archive.ubuntu.com/ubuntu/ precise main restricted universe multiverse
###### Ubuntu Update Repos
deb
http://br.archive.ubuntu.com/ubuntu/ precise-security main restricted universe multiverse
deb
http://br.archive.ubuntu.com/ubuntu/ precise-updates main restricted universe multiverse
deb
http://br.archive.ubuntu.com/ubuntu/ precise-backports main restricted universe multiverse
deb-src
http://br.archive.ubuntu.com/ubuntu/ precise-security main restricted universe multiverse
deb-src
http://br.archive.ubuntu.com/ubuntu/ precise-updates main restricted universe multiverse
deb-src
http://br.archive.ubuntu.com/ubuntu/ precise-backports main restricted universe multiverse
###### Ubuntu Partner Repo
deb
http://archive.canonical.com/ubuntu precise partner
deb-src
http://archive.canonical.com/ubuntu precise partner
###### Ubuntu Extras Repo
deb
http://extras.ubuntu.com/ubuntu precise main
deb-src
http://extras.ubuntu.com/ubuntu precise main
Pronto, salve e agora tente dar um upgrade, coloca essa linha toda no terminal;
sudo rm /var/lib/apt/lists/* ; sudo rm /var/lib/apt/lists/partial/* ; sudo apt-get clean ; sudo apt-get -f install ; sudo apt-get update
Se ainda der problemas com chaves, algum erro GPG no terminal vc instale esse pacote;
http://ppa.launchpad.net/nilarimogard/webupd8/ubuntu/pool/main/l/launchpad-getkeys/launchpad-getkeys_0.3.2-1~webupd8~oneiric_all.debinstale o deb e depois rode o comando no terminal;
sudo launchpad-getkeys ; sudo apt-get update
Sempre que houver problemas com chave, e so rodar esse comando novamente.