There are 2 ways:
1. To reinstate GRUB as the system bootloader it needs to be reinstalled into the MBR.
Boot the system from the Ubuntu Live CD and select “Try Ubuntu without any change to your computer”.
Open a Terminal session – Applications, Accessories, Terminal
To enter the GRUB configuration mode, type in “sudo grub” and press Enter. Then type in the following commands in sequence:
– root (hd0,0)
– setup (hd0)
Reboot the system. You’ll get the GRUB bootloader but Vista won’t be an option – we need to add this to the boot options.
Boot into Ubuntu and open up another Terminal session. Then, type in sudo gedit /boot/grub/menu.lst
Scroll down to the bottom of the file and type in the following text strings:
title Windows XP
Save the file and reboot. When the GRUB loader launches hit ESC for the boot menu. Windows XP is the last option – select it and XP will load.
2. Assume ubuntuHD is mounted on /media/ubuHD in the live system):
$sudo mount --bind /dev /media/ubuHD/dev
$sudo mount --bind /proc /media/ubuHD/proc
$sudo chroot /media/ubuHD
now redo your grub install.