It seems as though your GRUB was in the MBR overwritten by the new XP installation.:-(
You should have some bootable media however in order to restore it. Check this
link for a sample solution of your problem.
And i repeat - you need a rescue disk or a liveCD like KNOPPIX, whatever, from where you could run
/sbin/grub-install /dev/hd(x)
not very explanatory, but hope it helps!
Cheers