Windows XP likely overwrote the Master Boot Record on your hard drive, so you're currently stuck with the Windows bootloader (which does not recognize Linux). You will need to get a LiveCD and re-install RedHat's bootloader (GRUB).
Once your system has booted off of the LiveCD, open a root shell, mount the RedHat partition somewhere, copy the RedHat partition's /boot directory to the LiveCD's /boot directory (e.g. to ensure that the /boot/grub/menu.lst file is in the correct place), and then use grub-install to overwrite the Windows XP bootloader on your hard drive. Be careful to specify the GRUB configuration on the RedHat partition, and be sure to specify the correct disk on which to install the bootloader (the corresponding /dev file will NOT have a number after it).
For example: grub-install --root-directory=/path/to/RedHat/root/directory /dev/sda
Best of luck!