Impossible to get information from update manager

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http://askubuntu.com – When I press the button "check" to find updates, I always get this error: Failed to get information from repository W:Failed to fetch http://ppa.launchpad.net/dylanmccall/ppa/ubuntu/dists/precise/main/sourc... 404 Not Found, W:Failed to fetch htp://ppa.launchpad.net/dylanmccall/ppa/ubuntu/dists/precise/main/binary-amd64/Packages 404 Not Found, W:Failed to fetch http://ppa.launchpad.net/dylanmccall/ppa/ubuntu/dists/precise/main/binar... 404 Not Found, E:Some index files failed to download. They have been ignored, or old ones used instead. I've noticed that some (HowTos)