Everytime I have to look up how to install grub after the mbr got messed up. Therefor here a link to speed up the process…
Manual page of Grub about grub-install
The important part is the line
mount -t ext2 /dev/hda1 /mnt grub-install --root-directory=/mnt hda
[...] The Installation of grub is described in grub installation. [...]