To do it right, you need to do something like the following:
Boot using the install CD (there's no rescue option, but you can
do it yourself).
log in as root
mount the root partition, save a copy of your existing lilo.conf
mount /dev/hda1 /mnt
cp /mnt/etc/lilo.conf /mnt/etc/lilo.conf.rescue
set up LILO. I always use expert mode and install on the MBR,
but you can probably get away with simple mode.
copy your /etc/lilo.conf.rescue over your /etc/lilo.conf