If you are running Ubuntu version 11.04 (aka Natty Narwhal) then you, my friend, are using a relic - the Linux equivalent of Windows XP (eek!). The Ubuntu version that introduced the Unity interface is already "obsolete". By that I mean that the official support for this version is now finished, and to keep up with security and other updates, you should look to updating to 11.10 and thence to 12.04. From the official notification:
This note is just to confirm that the support period for Ubuntu 11.04
(Natty Narwhal) formally ends on October 28, 2012 and Ubuntu Security
Notices no longer includes information or updated packages for
The supported upgrade path from Ubuntu 11.04 is via Ubuntu 11.10
(Oneiric Ocelot). Instructions and caveats for the upgrade may be
found at https://help.ubuntu.com/community/OneiricUpgrades. Note
that upgrades to version 11.10 and beyond are only supported in
multiple steps, via an upgrade first to 11.10, then to 12.04.
Both Ubuntu 11.10 and Ubuntu 12.04 continue to be actively
supported with security updates and select high-impact bug fixes.