How can I fix a 404 Error using the Ubuntu archives?

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http://askubuntu.com – When running an update I get these errors: :Failed to fetch http://gb.archive.ubuntu.com/ubuntu/dists/karmic-backports/main/source/S... 404 Not Found [IP: 80] , W:Failed to fetch http://gb.archive.ubuntu.com/ubuntu/dists/karmic-backports/restricted/so... 404 Not Found [IP: 80] , E:Some index files failed to download. They have been ignored, or old ones used instead. There were more of the same kind of lines. How do I fix this? (HowTos)