The L99 error is a Lilo error. It means that Lilo didn't install correctly. To boot into Linux pop the first Slack CD into your CD-ROM drive, boot up, and at the boot prompt type in
Code:
linux root=/dev/hda2
Replace the /dev/hda2 with the location of your root partition. Now my limited knowledge doesn't know how to fix Lilo to install into the MBR, but I do know that
1. Linux needs a boot loader such as GRUB or Lilo
2. booting into DOS and doing fdisk /mbr will replace any attempt to install a boot loader onto the hard drive and make windows the only entry into the MBR,