Boot the Slackware CD, it should have instructions on the first page, eg:
Code:
boot: bare.i root=/dev/hda1 ro
Where /dev/hda1 is your Slackware installation.
Then try using grub instead of lilo (package at
http://www.linuxpackages.net)
Install using 'installpkg grub*.tgz'
And try running 'grubconfig'.