I've been trying to install 12.04 LTS onto a Dell with a Pentium III from CD.
Booting from the CD, the installer gets through the "Who are you?" process and begins copying files. The progress bar gets as far as the last period in "Copying files...". The box clears, and an error box comes up saying:
The installer has encountered an unrecoverable error. A desktop session will now be run so that you may investigate the problem or try installing again.
When I try to install from this desktop session, the installer gets to the same point, the copying files box closes, and then just stops. The pointer is busy, the CD drive spins up occasionally with no data transfer, and there's no hard drive activity.
When I boot from the CD and access the disk boot menu, the disk checks good and memory checks good (I upgraded the original memory to 512 MB). I also updated the BIOS to the newest version from Dell. This is an older L866r, but should meet the requirements.