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