I can no longer get past the windows logon screen, password was not changed.

While trying to setup my brother-in-laws Latitude D600 running XP Pro to get on my home network, I can no longer get past the Windows logon screen. The last thing that I clicked on in one of the wizards was a choice between the machine being used to connect to a domain at a business or to a home network. I chose the home network and was prompted to restart for the changes to take effect. When the computer rebooted, the login screen came up with his username but the password no longer works.