Finally, we tried to boot on a knoppix CD we got handy and found that we could install Debian the gory way : debootstrap from Knoppix and a local woody mirror, then chroot, lilo, etc. Unfortunately, something went wrong because the ide-scsi kernel module was loaded and lilo ended in error sending weird stuff to the IDE controller. Then the computer refused to boot either from a floppy, from a CD or from the hard disk ! Now we are puzzled and we can't even open the machine to see what's wrong since there is a seal that voids guarantee if broken. If someone has any clue about his issue, that would be very appreciated.