I have a PC with Win XP SP2. There are 2 users on the PC, each admin users, each with no password. When you start the PC, it displays the Welcome screen, and you can click on either user's icon and Windows logs you in without asking for a password. So far so good. However, if you a)try and switch users, or b)logout and try to log in as either user, Windows now asks for a user password. But there is no password configured! You have to restart the PC every time you want to switch users.
I've tried the following:
Creating and deleting passwords in the Users and Passwords control panel
Clearing the 'Require password when coming out of standby' setting in the Power Options control panel