1) Boot with the Slackware installation CD.
2) Mount the Linux filesystem(s).
3) Run/use the lilo command that is on the Linux filesystem. And use the "-r" option.
Example: Assuming your Linux is on /dev/hda2.
Run these commands from the prompt you get when booting with the Slackware installation CD.
mount /dev/hda2 /mnt
/mnt/sbin/lilo -r /mnt
Of course if your Linux is on more than one partition. You will also have to mount the other partitions in the proper sub-directories.
Specifically boot, dev, and etc.