How to reinstall grub for linux

I have windows XP home installed in my PC.
Recently I brought a new HDD and installed Redhat 9 in that. All was fine.
The grub loader gave options to choose between linux and windows.

After a few months I installed XP professional also onto a new partition
in the new HDD. Linux was present on another partition. After installing
windows the grub loader doesnt work anymore. I have only dual boot for
XP home & XP professional.

How do I reinstall the grub loader so i can boot linux also. I have the linux
cd's with me.