The repository problem can probably be fixed by editing your sources list,
but first I would try as previously suggested to fix apt. sudo apt-get -f
install
There is also sudo dpkg --configure -a
Between those two commands you can fix most problems.

What the first one does its fix broken packages and the second one will
rescue an aborted install that can happen if your computer locks or similar
thing prevents applications from completing the installation process.

If you know which repo is "bad" then you can use Synaptic to edit the
sources or sudo into Nautilus and navigate to etc/apt/sources.list