Fix Bootloader After Dual Boot Installation

The most common scenario, restoring Windows Vista or 7, is one of the easiest. Run the installation CD at startup, and after the location options select Repair Your Computer. Unselect your installation if the system locates it, and then open Command Prompt. Type in:
bootrec.exe /fixboot
Press enter, then type:
bootrec.exe /fixmbr
Close all windows, do a restart, and your Vista/7 bootloader should appear.
Those restoring XP have it even easier. Insert the install CD, allow it to proceed, then type r when it asks if you are repairing rather than installing. This brings up a command line, in which you should type, pressing enter between lines:
fixboot
fixmbr
exit
Finally, those restoring Ubuntu or a similar Linux system need to start by identifying the drive their system is installed on. From the terminal on an install disk, type:

sudo fdisk –l

From the information that this calls, identify the drive that holds the Linux system, such as /dev/sda2. Next, substituting in your own drive in place of sda2:
sudo mkdir /media/sda2
sudo mount /dev/sda2 /media/sda2
sudo grub-install –root-director=/media/sda2 /dev/sda
Source: ghacks.net