Cannot update on Ubuntu 11.10

Hello, I'm new to Ubuntu. I'm running Ubuntu 11.10 and have recently tried to update it by using the command:

sudo apt-get update

However, every time I run that command, I get this error message:

W: GPG error: http://ppa.launchpad.net oneiric Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 5A9A06AEF9CB8DB0W: Failed to fetch http://ppa.launchpad.net/minecraft-i...source/Sources 404 Not Found


W: Failed to fetch http://ppa.launchpad.net/minecraft-i...-i386/Packages 404 Not Found


E: Some index files failed to download. They have been ignored, or old ones used instead.


Please explain what the problem could be if you can.

Thank you!

P.S. This is my first post on the forums. If I posted this in the wrong place, please tell me! :)