This is a longshot, but it may save you a lot of time and heartache if it works. I'm assuming you have a fresh Kubuntu 5.10 installer CD, right? Try this.
Open up a terminal. Type in
Code:
sudo cp /etc/apt/sources.list /etc/apt/sources.list_backup
sudo gedit /etc/apt/sources.list
Then, paste this over the entire text in the file that opens up (don't worry--we already made a backup of the old file)
Code:
deb cdrom:[Ubuntu 5.10 _Breezy Badger_ - Release i386 (20051012)]/ breezy main restricted
Then, save that file and close it. Stick the Kubuntu 5.10 CD in your CD-ROM drive. From the terminal again, type
Code:
sudo apt-get update
. See if the update goes well. If it goes well (no errors returned), then type in
Code:
sudo apt-get dist-upgrade
If that goes well, then type in
Code:
sudo apt-get install kubuntu-desktop
Then reboot.