Retrive GRUB after installing WINDOWS

NOTE: linux is installed to boot in mode BIOS and WINDOWS(7) in mode bios too.

To repair grub you have to start linux on a live-CD or a live-USB.
KNOPPIX launches grub-legacy and can not restablish grub-pc (grub2) The file /boot/grub/stage1
is not read correctly on my laptop, but the live-usb I built with grub2 works flawless!

Start on a liveCD or liveUSB that has grub2 installed.
(or grub-legacy if your system use grub-legacy. In this case KNOPPIX works fine)

to see the partitions:

~$ sudo fdisk -l (or blkid)

If you use a liveUSB, the /mnt directory is free.
If you use a liveCD /mnt is busy, so use another free directory.
To be sure of having a free directory, let's create the mntgrub directory:

~$ sudo mkdir /mntgrub

Then, depending on the case (/mnt or /mntgrub) do:

~$ sudo mount /dev/sdxx /mntgrub

/dev/sdxx is the directory to use to repair grub which contains /boot/grub/... In my case it's /dev/sda5

Then do:

~$ sudo grub-install --root-directory=/mntgrub /dev/sda

/dev/sda designating the disk to repair in my case.
It works for me, hope it will work for you.

WARNING: If you accidentally corrupt the /boot/... directory, you will not be able to rebuild Grub.
So it's safe before you start, backing up the /boot directory