This is the message I get whenever I open Update-Manager.(Ubuntu Software Center doesn't open either).
An unresolvable problem occurred while initializing the package information.
Please report this bug against the 'update-manager' package and include the following error message:
'E:Encountered a section with no Package: header, E: Problem with MergeList /var/lib/apt/lists/extras.ubuntu.com_ubuntu_dists_precise_main_binary-i386_Packages, E:The package lists or status file could not be parsed or opened.'