Now, prepare your Ubuntu Live CD and boot your computer from it until the desktop appears. Once, the desktop is ready, open Terminal and type the following command:
. gksudo nautilus
Now, at the left sidebar, navigate to the root partition of your installed Ubuntu system, NOT the root partition of the Live CD.
For the example, I'll reset password of the user "hok00age". Open file "/etc/shadow" with your favorite text editor and then search for line containing the username:
Look at that code! All you need to do is replace the weird phrase "$1$2TUdk8Z0$tb2Fn6Idgo8dq9EgYv4xZ0" with another weird phrase "U6aMy0wojraho". If you did it well, you'll see the line is changed like the below one:
It will, replace your old password with newly BLANK password. So, if you want to log in your computer just leave the password field empty.
Now, reboot your computer and login with the BLANK password.
source : http://www.ubuntubuzz.com