Sure, if you installed them both on the same system (meaning the same hardware configuration) then you will run into ALOT less headaches, but here goes:
Boot into RH as root.
I am assuming you are using lilo, if not, these are for lilo so you can either use lilo and do this, or wait for a response for a grub configuration...
open /etc/lilo.conf in your favorite text editor, for me it's
pico /etc/lilo.conf
then scroll down to the bottom of the document.
There should be entries for RH already in there, if not, add something like:
image = /boot/vmlinuz
root = /dev/hdb1 (or wherever you have RH installed at)
label = RedHat
read-only
Ok, now that's just to be able to boot into RH, now below that add:
other = /dev/hda1
table = /dev/hda
label = Windows
and that will allow lilo to Windows as well.
At the top of the file, place a line:
default = RedHat
to by default have the better OS boot.
And then exit the editor and save the file.
Then run
/sbin/lilo
And something like this:
Adding
RedHat *
Windows
...Done
Should appear if it was done correctly.