I've been trying to upgrade my ubuntu 11.04 desktop computer, but when I click on the ugrade button:

I get this error:

I've tryied to change my repositories, but it changes nothing in the error((on the "setting new software channel").

Can someone point me in the right direction?

This is my sources.list:

# deb http://ppa.launchpad.net/ailurus/ppa/ubuntu karmic main # disabled on upgrade to karmic
# deb-src http://ppa.launchpad.net/ailurus/ppa/ubuntu karmic main # disabled on upgrade to karmic

# deb cdrom:[Ubuntu 9.04 _Jaunty Jackalope_ - Release i386 (20090421.3)]/ jaunty main restricted
# See http://help.ubuntu.com/community/UpgradeNotes for how to upgrade to
# newer versions of the distribution.

deb http://us.archive.ubuntu.com/ubuntu/ natty main restricted multiverse universe

## Major bug fix updates produced after the final release of the
## distribution. deb http://us.archive.ubuntu.com/ubuntu/ natty-updates main restricted multiverse universe

## N.B. software from this repository is ENTIRELY UNSUPPORTED by the Ubuntu
## team. Also, please note that software in universe WILL NOT receive any
## review or updates from the Ubuntu security team.

## N.B. software from this repository is ENTIRELY UNSUPPORTED by the Ubuntu 
## team, and may not be under a free licence. Please satisfy yourself as to 
## your rights to use the software. Also, please note that software in 
## multiverse WILL NOT receive any review or updates from the Ubuntu
## security team.

## Uncomment the following two lines to add software from the 'backports'
## repository.
## N.B. software from this repository may not have been tested as
## extensively as that contained in the main release, although it includes
## newer versions of some applications which may provide useful features.
## Also, please note that software in backports WILL NOT receive any review
## or updates from the Ubuntu security team. deb-src http://pt.archive.ubuntu.com/ubuntu/ jaunty-backports main restricted universe multiverse

## Uncomment the following two lines to add software from Canonical's
## 'partner' repository.
## This software is not part of Ubuntu, but is offered by Canonical and the
## respective vendors as a service to Ubuntu users. deb http://archive.canonical.com/ubuntu natty partner deb-src http://archive.canonical.com/ubuntu natty partner

deb http://us.archive.ubuntu.com/ubuntu/ natty-security main restricted multiverse universe deb http://us.archive.ubuntu.com/ubuntu/ natty-proposed restricted main multiverse universe
# deb http://deb.torproject.org/torproject.org karmic main # disabled on upgrade to maverick
# deb-src http://deb.torproject.org/torproject.org karmic main # disabled on upgrade to maverick deb http://extras.ubuntu.com/ubuntu natty main #Third party developers repository