No puedo actualizar ni instalar desde el centro de software de ubuntu

hola a todos:
He actualizado la versión de ubuntu a la 11.10 y no me funciona el gestor de actualizacones. Me sale el siguiente problema. Espero que me podais ayudar.

W:GPG error: http://packages.medibuntu.org oneiric InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 2EBC26B60C5A2783, W:GPG error: http://es.archive.ubuntu.com oneiric-backports Release: The following signatures were invalid: BADSIG 40976EAF437D05B5 Ubuntu Archive Automatic Signing Key <ftpmaster@ubuntu.com>, W:Failed to fetch gzip:/var/lib/apt/lists/partial/es.archive.ubuntu.com_ubuntu_dists_oneiric_universe_source_Sources Hash Sum mismatch
, W:Failed to fetch gzip:/var/lib/apt/lists/partial/es.archive.ubuntu.com_ubuntu_dists_oneiric_main_source_Sources Hash Sum mismatch
, W:Failed to fetch gzip:/var/lib/apt/lists/partial/es.archive.ubuntu.com_ubuntu_dists_oneiric_multiverse_source_Sources Hash Sum mismatch
, W:Failed to fetch bzip2:/var/lib/apt/lists/partial/es.archive.ubuntu.com_ubuntu_dists_oneiric_restricted_source_Sources Hash Sum mismatch
, W:Failed to fetch gzip:/var/lib/apt/lists/partial/es.archive.ubuntu.com_ubuntu_dists_oneiric_universe_binary-i386_Packages Hash Sum mismatch
, W:Failed to fetch bzip2:/var/lib/apt/lists/partial/es.archive.ubuntu.com_ubuntu_dists_oneiric_restricted_binary-i386_Packages Hash Sum mismatch
, W:Failed to fetch bzip2:/var/lib/apt/lists/partial/es.archive.ubuntu.com_ubuntu_dists_oneiric_multiverse_binary-i386_Packages Hash Sum mismatch
, W:Failed to fetch bzip2:/var/lib/apt/lists/partial/es.archive.ubuntu.com_ubuntu_dists_oneiric_main_binary-i386_Packages Hash Sum mismatch
, E:Some index files failed to download. They have been ignored, or old ones used instead.