Sure enough, Windows will wipe out your existing MBR and put whatever it wants there.
After installing Windows, you will need a way to boot into Red Hat. In SuSE, you can simply use the install media. It would be nice and easy if you could do the same in Red Hat. Because then all you have to do is run
and grub will be reinstalled to your MBR.
If you can't boot from the installation media, I would suggest booting with something like knoppix. Then mount the partition that your / filesystem is on to some mount point. Then chroot on that mount point. Then same old deal: