You should be able to boot from the Fedora install disk.. Once it boots up, do ctrl-alt-F1 to get a command-line (terminal mode)
We need to know what partition the /boot directory is on. Let's assume hdb1. We're also assuming that you have 2 IDE/PATA drives--hda and hdb
To install GRUB, do this:
grub starts the grub shell
root (hd1,0) tells grub that /boot is on drive 2, partition 1 (grub counts from zero)
setup (hd0) puts grub on the mbr of drive 1
If you cannot get a command line using the install disk, you can do the above from any "live CD" linux distro.