You should be able to boot from the Redhat install disk and get into you Linux from there. Since the grub config file is already setup, all you need to do is get grub back in the mbr.
When the install CD boots up, see if you can get to a terminal using ctrl-alt-F1. If so, then do this:
(assumes IDE disk with Windows on partition 1, and Linux on 2)
grub starts the grub shell
root (hd0,1) tells grub that /boot is here (note that grub counts from zero)
setup (hd0) puts grub on the mbr