If you have an existing Linux machine (already running GRUB) and you want to install a fresh version of Ubuntu on it, this page may be handy. All you need to do is download a kernel and an initrd file, reboot and issue a couple of GRUB command lines, and you can install everything else over the network from the Ubuntu repositories.
I’ve just got a new hosted server which came with 6.06 installed, and I wanted to wipe it and start with a clean 8.04. This was a very quick and easy way to do it, especially since I didn’t have easy access to the machine’s CD/DVD drive.