Repair Ubuntu installation

if you can run the ifconfig utility (as root), and have a single ethernet adapter which can use DHCP to access the network:

sudo ifconfig eth0 up
sudo dhclient eth0 &

That should activate your first ethernet adapter and run dhclient to get it an address via DHCP. If that works successfully, you might be able to do repair your installation by running:

sudo aptitude install ubuntu-desktop

That command will install the packages which are included in a base Ubuntu install. (you can substitute "kubuntu-desktop" if you use KDE or "xubuntu-desktop" if you use XFCE.

If you cannot get networking at all, the best bet is as Justin suggested above, use apt-cdrom to install the ubuntu-desktop package from your CD.

You can configure apt to use the install cd as its repository, and then install what you need for net access, and gnome, from the cd.

sudo apt-cdrom -d <mountpoint> add is the command you want for this. make sure the mountpoint is the cd-rom mountpoint listed in your /etc/fstab.