xp overwrote the bootloader links in the mbr.
get a live cd and just re-link the mbr to your /boot partition
eg if /boot is on partition 5
while in live mode...mount the harddrive partition and edit grub's /boot/grub/menu.lst to have a chainloader menu
my signature may help?
if you do not know how to use a live cd...too well..after doing just the grub...reboot to hard drive...boot into linux and then edit that file with root powers..the file /boot/grub/menu.lst