How can I reset my Windows XP password using a LiveCD?

Get a 32-bit Ubuntu 10.04 LTS LiveCD, and boot the computer from it.

Then follow these steps once in the LiveCD environment:

1. Go to Administration > Software Sources and enable the Universe and Multiverse software repository sources. Click Close, then Reload when prompted.

2. Either install chntpw from Synaptic Package Manager, or open a terminal and run:

   sudo apt-get install chntpw
   

   The latter method may be preferable, since this is a commandline program.

3. Mount the Windows XP partition by going to the Places menu in the top panel, and mounting the relevant filesystem by clicking it. It will be mounted once the Nautilus window for it opens.

4. In a terminal, type

   cd /media/
   

   and hit Tab to autocomplete with the name of your Windows partition. Then (for Windows XP) complete the rest of the path as WINDOWS/system32/config/. Hit Enter to change to this directory.

5. Run

   sudo chntpw -u username SAM
   

   where username is your actual Windows XP username. (If you don't get it right the first time, you can see a list of found users in the output and re-enter it with the right username.) Once it presents you with a text menu, enter 1 to clear the user's password.

Now just restart your machine and log in as the selected user, who will not have a password. Use Windows' own account management tools to set a new password if desired.

(Source)