This same exact thing happens within our domain. It only has happened on our XP machines so far ( have XP Pro and 2000 Pro clients). The username and password fields are "grayed out" (disabled) and the computer has to be rebooted before a user can log in. It doesn't happen that often, but it has happened enough that we Googled for a cause and solution. Nothing was found thus far to solve this issue for us. All of our users do have a password, and the computers will lock once the screensaver runs.