booting with wrong grub.conf

[John Cox]

I have an extra partition that I use to test other distributions. My
normal procedure is install but not let the new OS install grub. I
then manually modify my FC3 grub.conf. I didn't do this with an
installation of caos so now when I boot I'm using the grub.conf on the
caos partition. I don't want this as FC3 is my main OS. I tried using
FC3 rescue to reinstall grub to /dev/hda but that doesn't do the job. 
So what determines which grub.conf the system uses and how do I change
that?
Thanks.
John