11.10 - Can't install vlc and other software : packages not coming from authenticated sources

After installing Ubuntu 11.10 last week I couldn't install any mp3 or videoplayers such as vlc or any others. They show me something about a failure to install packages, and when I click on details I see this:

The action would require the installation of packages from not authenticated sources: libdvdread4 libvlc5 libvlccore4 vlc vlc-data vlc-nox vlc-plugin-notify vlc-plugin-pulse