Boot from live CD.
Mount the / partition.
Chroot to the mounted partition
Assign a password to your account to activate it.
You’ll want to boot from your Ubuntu Live CD, choosing “Try Ubuntu without any change to your computer” from the boot menu.
Once the system boots, open up a new Terminal window from Applications \ Accessories and then type in the following command:
sudo fdisk -l
This command is used to tell what device name the hard drive is using, which in most cases should be /dev/sda1, but could be different on your system.
Now you’ll need to create a directory to mount the hard drive on. Since we’re actually booting off the live cd, the directory doesn’t really get created anywhere.
sudo mkdir /media/sda1
The next command will mount the hard drive in the /media/sda1 folder.
sudo mount /dev/sda1 /media/sda1
Now it’s time for the command that actually does the magic: chroot.
sudo chroot /media/sda1
Now you should be able to use the passwd command to change your user account’s password, and it will be applied to the hard drive since we are using chroot.
Note that you’ll have to type your username after the passwd command in order to change the right password.
Now you should be able to reboot your system and log yourself in with your new password.