Yep... when you re-installed Windows it will have installed a Windows bootloader in the MBR hence erasing GRUB - thats why the multiboot menu has gone - did you also completely wipe over the Linux install when you re-installed Windows? - you could always try booting off the 1st Red Hat cd-rom and typing
at the boot prompt:
For more info check out:
......... maybe you've got the bootdisk?
get to a prompt and
(if on /dev/hda - modify accordingly if different)