Windows 7 Dual Boot Between Vista win7 and Ubuntu

[WhoCare]

My computer has used vista and Ubuntu. Then I install win7 and I can't enter Ubuntu. It only can switch between Vista and win7. But I want to switch between there of them. Somebody help me

 

[dyrer]

Boot with Ubuntu Live CD
in terminal type
(If your ubuntu installation is on sda3)

mkdir /media/mnt
mount /dev/sda3 /media/mnt/
mount -o bind /dev/ /media/mnt/dev
chroot /media/mnt => We are in the disk that we want to install grub
Then:
grub-install /dev/sda