This question already has an answer here:
Updater keep saying:
Failed to download repository information. Check your Internet connection.
W:Failed to fetch http://ppa.launchpad.net/screenlets/ppa/ubuntu/dists/quantal/main/source/Sources 404 Not Found, W:Failed to fetch http://ppa.launchpad.net/screenlets/ppa/ubuntu/dists/quantal/main/binary-amd64/Packages 404 Not Found, W:Failed to fetch http://ppa.launchpad.net/screenlets/ppa/ubuntu/dists/quantal/main/binary-i386/Packages 404 Not Found, E:Some index files failed to download. They have been ignored, or old ones used instead.