Ubuntu 8.10 "Intrepid Ibex" reached EOL (End of Life) on April 30 2010 as you can see here; https://wiki.ubuntu.com/Releases
. That's why you can't install packages.
Here's a guide for EOL distros upgrade; https://help.ubuntu.com/community/EOLUpgrades
To configure your repositories (Software sources) you can open Synaptic Package Manager and do it from there (under Settings).
You may want to install the newest version of Ubuntu which is version 10.10 "Maveric Meerkant". You can get it here
. All you need to do is burn the iso image to CD and boot from it.