Grub is a Linux bootloader.

Before installing Windows (backup of original MBR):

sudo dd if=/dev/sda of=sda.linux.mbr bs=512 count=1

After installing Windows (to have a backup):

sudo dd if=/dev/sda of=sda.windows.mbr bs=512 count=1

Boot from recovery CD/USB, mount /home, navigate directory where the MBR backups are stored.

sudo dd if=sda.linux.mbr of=/dev/sda bs=512 count=1

(Complex solution, as above way using dd works just fine) chroot http://wiki.ubuntuusers.de/GRUB#Methode-3-Chroot-ueber-ein-Live-System