It sounds as though grub was not installed at all (or at least not installed on the master boot record) -- or perhaps you installed Win98 or XP after Linux (which would have replaced grub with a Windoze loader). - I believe you will need to boot using your RedHat CD, and go into rescue mode. From there, you can boot into your Linux installation, review (and perhaps modify) the grub.conf file, and reset your bootloader to grub.
This article
http://kb.redhat.com/view.php?eid=94 gives some basic instructions for using rescue mode; a post by PVC9 in the thread
http://forums.techguy.org/showthread...hreadid=115840 also mentions how to re-install grub from rescue mode.
Hope this helps.