cd to /etc/apt, and edit your sources.list file to uncomment out the
last two lines of the comment which are normally:
# deb http://archive.ubuntu.com/ubuntu hardy universe
# deb-src http://archive.ubuntu.com/ubuntu hardy universe

You should then be able to launch the Synaptic Package Manager and
Reload which should help.

Check the Settings>Repositories in Synaptic Package Manager for the
Desktop version. Also, by setting it to the Main server with the
Desktop and Reloading, afterwards copy the sources.list files and
perhaps tar the entire directory from /etc/apt, and place it onto your
system that is not the Desktop - which may help your situation.

I've got the point, but not yet have a time to fix that..
By the way, to edit sources.list, can't use 'gedit', maybe 'vim'...
Is it right?

Try: $ sudo gedit <filename>
or $ sudo vi <filename>