Get Windows ubuntu dual-system back

After I reinstalled WindowsXP, the dual-boot menu is gone. Use ubuntu live CD to start the system and open a terminal
try

sudo fdisk -l

first to get the list of all the partitions and my linux is at hda3,

sudo -s
cd /
mkdir disk
mount /dev/hda3 disk
chroot /disk
grub-install /dev/hda
(some output here)
exit