as mucks sugested try booting in safemode.
in safemode you will be presented with the familiar logon screen, where you should be able to logon with the default "administrator" account (assuming this hasn't been changed*) once logged in click start then run and type "control userpasswords2" from this dialogue box you have the option to disable autologon or require a password on logon.
Although this should stop windows automatically logging on there appears to be a issue with logging on full stop which you would need to investigate in safe mode.
* Always a good idea to change default admin account and select a complicated password.