Well, you have two posible options:
1) With a LiveCD for example, boot in console form and you can use Grub to rewritte the MBR
Code:
grub-install --root-directory=/boot /dev/hda
Or with Lilo
Code:
chroot /mnt/hd /sbin/lilo -v -v
2) With a rescue disk of win using
Bye.....