I was running Ubuntu 7.10 on my server, but wanted to reinstall it with Debian etch. The machine in question doesn’t have a CD/DVD-ROM drive, and I couldn’t be bothered to set up a tftp server on my laptop so I could netboot it. The answer? Set up the existing grub to boot the network installer kernel and initrd (initial RAM disk).
The files you need are here and here. (You’ll need some different files if you don’t want the x86 version, that is left as an excercise for the reader)
Just stick them into your /boot/, configure /boot/grub/menu.lst with something like this:
title Debian etch installer
root (hd0,0)
kernel /boot/linux vga=normal
initrd /boot/initrd.gz
Since the initrd contains everything you need including the installer and network driver modules, and only gets read once (when booting), you can of course overwrite the partition that these files are stored on (though if you mess up the installation it’ll be a bit harder to get it re-started ;)