Ei, ekurth, abra o Terminal e digite:
sudo gedit /etc/apt/sources.list
Quando o arquivo sources.list for aberto você deve substituir as informações contidas nele pelo conteúdo abaixo:
# Automatically generated sources.list
# http://www.ubuntulinux.nl/source-o-matic
#
# If you get errors about missing keys, lookup the key in this file
# and run these commands (replace KEY with the key number)
#
# gpg --keyserver subkeys.pgp.net --recv KEY
# gpg --export --armor KEY | sudo apt-key add -
# Ubuntu supported packages (packages, GPG key: 437D05B5)
deb http://archive.ubuntu.com/ubuntu dapper main restricted
deb http://archive.ubuntu.com/ubuntu dapper-updates main restricted
deb http://security.ubuntu.com/ubuntu dapper-security main restricted
# Ubuntu supported packages (sources, GPG key: 437D05B5)
deb-src http://archive.ubuntu.com/ubuntu dapper main restricted
deb-src http://archive.ubuntu.com/ubuntu dapper-updates main restricted
deb-src http://security.ubuntu.com/ubuntu dapper-security main restricted
# Ubuntu community supported packages (packages, GPG key: 437D05B5)
deb http://archive.ubuntu.com/ubuntu dapper universe multiverse
deb http://archive.ubuntu.com/ubuntu dapper-updates universe multiverse
deb http://security.ubuntu.com/ubuntu dapper-security universe multiverse
# Ubuntu community supported packages (sources, GPG key: 437D05B5)
deb-src http://archive.ubuntu.com/ubuntu dapper universe multiverse
deb-src http://archive.ubuntu.com/ubuntu dapper-updates universe multiverse
deb-src http://security.ubuntu.com/ubuntu dapper-security universe multiverse
Depois, salve e feche o arquivo. Volte ao Terminal e digite:
sudo apt-get update