Vou apenas citar as respostas de pjc50 em unix.SE :
Some people have been able to get NTLDR to chain to grub: http://stringofthoughts.wordpress.com/2009/04/27/booting-linux-from-ntloader-windows-bootloader/
Although in practice it's easier to use a live CD. I usually do something like:
mount /dev/sda2 /mnt/somewhere # mount the linux partition chroot /mnt/somewhere bash mount /proc grub-install /dev/sda2