I believe WindowsXP cd can be used to boot the machine and use "fixmbr" to reload the MBR so you can boot into WindowsXP. I suspect that RH has a similar capability. See below where I did a similar thing with MDK9.1.
The later versions of MDK (9.0 & 9.1) and probably earlier versions use LILO and not GRUB. RH uses GRUB. I like both GRUB and LILO as boot loaders but each has strengths and weaknesses. Here is a good link which discusses common Linux Bootloaders:
http://www.xs4all.nl/~lennartb/bootloaders/
I recently upgraded my experimental machine from Win98se & MDK8.0 to WindowsXP & MDK9.1.
It was quite an experience! I installed MDK9.1 first and it went well. Upgrading Win98se to WindowsXP, of course wiped out the LILO on the MBR and put it's own loader in which never recognizes other OpSys on the box. I was able to boot from the MDK9.1 cd and use the "rescue" option and recreate/reload the LILO and update the MBR. Now I have dual boot with WindowsXP and MDK9.1