Hi,
Simply chroot into your existing installation from a liveCD, and install grub manually
Code:
#mount /dev/hdxy /mnt/***
#chroot /mnt/*** /bin/bash
#grub
>root (hdx,y)
>setup (hdx)
>quit
replace *** by the folder to which you mounted the root partition of the existing installation
replace x by the hard disk number/letter (the first being 0/a)
replace y by the partition number (the first being 0)