You will need a Live CD to recover GRUB after windows installation. I assume that you are going to recover an UBUNTU Box. Boot the Live CD and go to the terminal. And continue by entering:

$ sudo fdisk -l

This will ask your password, just enter it and press enter key. It will give you output like this:

Device Boot      Start         End      Blocks   Id  System
/dev/sda1   *           1          13      102400    7  HPFS/NTFS
Partition 1 does not end on cylinder boundary.
/dev/sda2              13        6768    54256640    7  HPFS/NTFS
/dev/sda3            6768       38913   258207585    f  W95 Ext’d (LBA)
/dev/sda5            6768        9318    20480000   83  Linux
/dev/sda6           22372       38913   132873583+   7  HPFS/NTFS
/dev/sda7            9319       22371   104848191    7  HPFS/NTFS

Now i will mount Linux (sda5 here), i have no external boot partition as you can see.(IF YOU HAVE external one, do not forget to mount it! )

$sudo mount /dev/sda5 /mnt
$sudo mount --bind /dev /mnt/dev
$sudo mount --bind /proc /mnt/proc

Now chroot into the enviroment we made :

$sudo chroot /mnt

After chrooting, you do not need to add sudo before your commands because from now, you will run commands as root.

You may want to edit /etc/default/grub file to fit your system (timeout options etc)

#nano -w /etc/default/grub

Play with the options if you want.(But do not forget to give grub-update command if you saved it )

Now install/recover Grub2 via :

#grub-install /dev/sda

command.However you may get errors with that code like me.If so please use this command :

#grub-install --recheck /dev/sda

Now you can exit the chroot, umount the system and reboot your box :

#exit
$sudo umount /mnt/dev
$sudo umount /mnt/proc
$sudo umount /mnt
$sudo reboot

Source: http://www.ubuntu-inside.me/2009/06/howto-recover-grub2-after-windows.html