Try reinstalling grub, but this time at the shell from the 1st RHEL CD in rescue mode, try these four commands below (as root) to get you into a grub session first, in the example below I'm assuming your RHEL /boot partition is /dev/hda8, adjust accordingly. Remember grub counts from 0.
Grub error 17:
17 : Cannot mount selected partition
This error is returned if the partition requested exists, but the filesystem type cannot be recognized by GRUB.