First :

Insert Ubuntu Boot cd and click on “Try Ubuntu without any change to your computer”

After booting open the terminal and run

sudo fdisk -l

Now you need to create mount directory coz your booting off the live cd, the directory doesn’t really get created anywhere.

sudo mkdir /media/sda1

Now you need to mount

sudo mount /dev/sda1 /media/sda1

Now you need to run chroot.

sudo chroot /media/sda1

after this run

chroot username

it will ask for

new password enter

after wards boot normally with out ubuntu disk and login with new password

sudo chroot /media/sda1