I bumped into the problem that I can't download any updates or new packages. I had this problem before which I could solve through several steps
sudo rm /var/lib/apt/lists/* -vf
sudo apt-get clean
sudo apt-get update
This gives me still a
W: Failed to fetch
http://ucmirror.canterbury.ac.nz/ubuntu/dists/quantal-security/multiverse/binary-i386/Packages
404 Not Found
E: Some index files failed to download. They have been ignored, or old
ones used instead.
I tried a different server by hand, and get a 'No suitable download server found' when I ask Ubuntu to find one for me. I am not behind a firewall, there is no proxy server set up in the settings or anything like that.