so you installed GRUB into MBR or what?
man, the first mistake one can do if there's windows on the same HD..ok, enough of joking. There should be the Grub RPM package(s) on some of the redhat cds, altough I don't know on which one...you'll just have to search. but first things first.
make a bootable floppy so that you can boot your linux after the reinstallation of windoze..RH has some GUI tool for that I think. after that do the reinstallation of windoze, boot from the floppy, mount cdroms, search for an rpm-package for GRUB (like grub-x.xx.rpm where x.xx is the version number) and install it. the installation was something like
rpm -ivh grub-whatever.rpm
rpm -Uvh grub-whatever.rpm
I'm not sure about the rpm command...but if you use GUI, double-clicking the file should do.