When we have both Windows and Linux in one computer (in dual OS mode)... If you install XP again, the Linux's boot loader will be overwrittn. In those cases you may need to recover the GRUB again.
To recover the GRUB follow these steps (These were helped me, hopefully to you too :) )
1. Put your RedHat CD 1
2. In the linux prompt type "linux rescue"
3. Follow the instructions (You may need to select language and various other things) - It also search for the existing Linux installation and let you know
4. You will end up with ‘sh#’ prompt. There type "chroot /mnt/sysimage/" and enter
5. Then type "grub-install /dev/hda"
6. Type "reboot"
These should work with RedHat family distribution. But it may also work with other distributions too.
Thats all. Now your Linux boot loader should work and you should be able to go to Linux and Windows.