1. boot a live disk and open a terminal
2. "sudo -i" to become root
3. "fdisk -l" to see where your linux root partition is
4. "mount /dev/hda1 /mnt" assuming your root partition is on /dev/hda1
5. "mount -o bind /dev /mnt/dev" and
6. "mount -t proc /proc /mnt/proc" to get some system information
7. chroot in the new system with "chroot /mnt" and reinstall grub with
8. "grub-install /dev/hda"
9. do an "update-grub" and you should be good
10. now exit and unmount what you just mounted with
exit
umount /mnt/proc
umount /mnt/dev
umount /mnt
11. and reboot