I have now done it a couple of times, but always have to look it up. Here are the steps to re-install Grub from a live Ubuntu CD, when you have Windows and Linux on your hard disk. You must be root, or run these commands with
mount /dev/sda2 /mnt
mount /dev/sda3 /mnt/boot, if you have a separate boot partition
mount -o bind /dev /mnt/dev
mount -o bind /sys /mnt/sys
mount -t proc /proc /mnt/proc
chroot /mnt /bin/bash
Above commands are from method 3 in GRUB (in German).