Mandrive took over as the master booter (made that term up). You will have to boot into mandriva, then mount the volume that has the /boot directory for fedora core. First login to a console (alt-f2 -> konsole if using KDE). I'll make a rather broad assuption that /dev/hda1 is XP, /dev/hda2 is fedora core, /dev/hda3 is swap, and /dev/hda4. You'll have to verify that.
user@comp> su -
root@comp# mkdir -p /mnt/fc4_root
root@comp# mount /dev/hda2 /mnt/fc4_root
root@comp# less /mnt/fc4_root/boot/grub/menu.lst
now look at the title for the main fc boot (this is a slightly edited Kubuntu, but should be close). Now we want to make note of the path to which kernel you want to boot off of.
title Fedora Core 4
kernel /boot/vmlinuz-2.6.15-fc4 root=/dev/hda2 ro quiet splash
Now we will edit the Mandriva /etc/lilo.conf. Substitute vi for you favorite text editor (kwrite, gedit, pico, emacs...)
root@comp# vi /etc/lilo.conf
insert the following at the end of the:
label=Fedora Core 4
Now just run lilo to re-write MBR with new boots.
Fedora Core 4
Hopefully everything goes smoothly. There is a gui in drakconf, but I find the command line easier to explain.