put in the fedora cd, and boot to rescue mode, scan for existing installation and boot. once in at a prompt run
# chroot /mnt/sysimage
# grub-install <device> (e.g. /dev/hda)
and you're done. you can install in whatever partition you want as you may have another bootloader in the MBR which can be chained on from, e.g. vista boot loader. i hink it's possible to chain load on the xp too, but generally it's always so much nicer to chain from grub to xp than any other layout.