when i check for updates the message is "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 , E:Some index files failed to download. They have been ignored, or old ones used instead." please suggest what i should do