Ubuntu 12.04
gnome environment.

In terminal I downloaded virtualbox key
sudo apt-key add oracle_vbox.asc
But did not finish the installation of virtualbox.
Virtualbox 4.1.12_ubuntur77245.
was already installed, using software centre.

When running update manager I now get this error message:W:Failed to fetch http://download.virtualbox.org/virtualbox/
debian/dists/precise/InRelease Unable to find expected entry
'contrib/source/Sources' inRelease file (Wrong sources.list
entry or malformed file)
,E:some index files failed to download. They have been ignored,
or old ones used instead.

How can I end the error messages?

Thank you.