All the little houses falling over just right

To boot CentOS 5, all the following needs to be set up and running BEFORE you reboot your computer.

1. GRUB installed on the boot drive (aka sda, ata1, sata1);
2. a /boot that grub can find and read. In particular, it must be small and at the front of the boot drive;
3. a kernel+initrd that contains the drivers to find and mount the final root filesystem (aka /); this can involve swearing, repeated applications of mkinitrd. At worse, you can pull initrd apart with gzip+cpio, add the drivers, modify init and repack it. Note that this is not a shell script but a nash script. CentOS 6 has moved to dash;
4. a / that has a working init and all that entails (obviously);
5. an /etc/fstab that won't fail out because fsck -A can't find a drive or partition. If you've moved / to another partition or to LVM, then you need to update fstab. You can set the last 2 columns to 0 if you don't want fsck to check that mount.