Since we had two disks in the server, and the other one was unused (simply mounted on /backup), I repartitioned it, installed new Debian GNU installation on it (using debootstrap), compiled the kernel, and successfully rebooted into it on the first try!
Then I went to move the installation to /dev/hda, thereby replacing the stock setup we got from InterServer. Evertyhing worked fine, and then I went to set up software RAID-1. I made a minor mistake in lilo.conf, and the server didn't want to boot properly. We filled in a Support Ticked at InterServer's site, and in 2 hours, John Quaglieri booted the system from the CD, fixed lilo.conf and even performed one additional step regarding RAID-1. I was impressed by his support. --DavorOcelic