Thanks to Howard Ball for this:

Here's what to do if your RedHat crashes and you have no working rescue disk:

First, boot the system with something like tomsrtbt or the linux bootable-business-card (links below).

Then mount your filesystem somewhere sane (I'll assume /dev/hda1 is your root / and /dev/hda2 is /boot...yours are probably something different. You might fdisk -l /dev/hda to remember how you partitioned.

Make a directory and mount your filesystems like this:
mkdir /rescuemount
mount /dev/hda1 /rescuemount
mount /dev/hda2 /rescuemount/boot

Then chroot yourself into your hard drive's environmment like this:
chroot /rescuemount /bin/bash

Now put a floppy in the floppy drive and:
/sbin/mkbootdisk

This should generate a duplicate of your boot disk. So type 'exit' to get out of the chroot'ed environment.

Unmount your filesystems (umount /dev/hda1; umount /dev/hda2
Reboot the system with your new rescue disk.

You can get Tom's rtbt at www.toms.net/rb/
and the Bootable Business Card at www.lnx-bbc.org/

These two tools are invaluable when repairing machines that won't boot for some reason. Every serious Linux user should become familiar with them...before they need them.