de nuevo error al actualizar Ubuntu 12.04 LTS

me sale el maldito triangulo rojo otra vez,y intento actualizar y dice q no hay actualizaciones disponibles, le doy comprobar y me sale esto :S, q hago ???

W:GPG error: http://packages.medibuntu.org precise InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 2EBC26B60C5A2783, W:GPG error: http://ftp.heanet.ie lucid-getdeb Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY A8A515F046D7E7CF, W:Failed to fetch cdrom://Ubuntu 12.04 LTS _Precise Pangolin_ - Release i386 (20120423)/dists/precise/main/binary-i386/Packages Please use apt-cdrom to make this CD-ROM recognized by APT. apt-get update cannot be used to add new CD-ROMs
, W:Failed to fetch cdrom://Ubuntu 12.04 LTS _Precise Pangolin_ - Release i386 (20120423)/dists/precise/restricted/binary-i386/Packages Please use apt-cdrom to make this CD-ROM recognized by APT. apt-get update cannot be used to add new CD-ROMs
, W:Failed to fetch cdrom://Ubuntu 11.10 _Oneiric Ocelot_ - Release i386 (20111012)/dists/oneiric/main/binary-i386/Packages Please use apt-cdrom to make this CD-ROM recognized by APT. apt-get update cannot be used to add new CD-ROMs
, W:Failed to fetch cdrom://Ubuntu 11.10 _Oneiric Ocelot_ - Release i386 (20111012)/dists/oneiric/restricted/binary-i386/Packages Please use apt-cdrom to make this CD-ROM recognized by APT. apt-get update cannot be used to add new CD-ROMs
, W:Failed to fetch http://ftp.usf.edu/pub/ubuntu/dists/precise-proposed/Release Unable to find expected entry 'universedeb/binary-i386/Packages' in Release file (Wrong sources.list entry or malformed file)
, W:Failed to fetch http://ppa.launchpad.net/leolik/leolik/ubuntu/dists/precise/main/source/... 404 Not Found
, W:Failed to fetch http://ppa.launchpad.net/leolik/leolik/ubuntu/dists/precise/main/binary-... 404 Not Found
, W:Failed to fetch http://ppa.launchpad.net/tldm217/tahutek.net/ubuntu/dists/precise/main/s... 404 Not Found
, W:Failed to fetch http://ppa.launchpad.net/tldm217/tahutek.net/ubuntu/dists/precise/main/b... 404 Not Found
, E:Some index files failed to download. They have been ignored, or old ones used instead.