>Suppose you need to install windows 7 on a system in which Ubuntu 10.04 is already running. After installing windows you will found that your Grub2 boot loader not coming up. What you have to do is to reinstall Grub2 boot loader. For that run Ubuntu directly from cd and execute following command in terminal
$sudo mkdir /a
$sudo mount /dev/sda2 /a
$sudo grub-install –root-directory=/a /dev/sda
In above given command /dev/sda2 is boot partition and /dev/sda is hard disk where grub2 need to be installed. You can change hard disk (dev/sda) and boot partition (/dev/sda2) according to your setup.