First of all... you need a DOS boot disk, not hard to find
Then, boot in DOS, and make a fdisk /mbr.
When you are done with that, boot Slack from the CD, read carefully what it says to succesfully boot the partition where you got Slack installed (usually /dev/hda1), and when you got prompt, do a pkgtool.
In pkgtool, rerun the LILO script, and install it in the first option.
Add the partitions you want... and that should do it