Use a RedHat CD to boot into Rescue mode. Once you're at a bash prompt, you need to figure out where your old root is and mount it somewhere. Create a directory for it, BTW. You might need to find and mount /boot, too, but do it under your newly mounted root.

Once you've mounted it and confirmed that /etc and /boot are where they should be (relative to their proper root), you can do a lilo -r /wherever/you/mounted/root and it will fix it all up.

Wade.