Although it's simple to allow the computer to automatically load to your main administrative user, if something goes wrong with that user's profile, you're sunk.. In you case, that may be exactly what happened.. Something may have become corrupted in the user's profile and it's not behaving as it should.. Or in some cases, a setting may have changed, such as a new password was created for the user, or the setting for Control Panel-User Accounts-Change the way users logon or off-"Welcome Screen" may have been altered.. (Or open a Run line, type "control userpasswords2" and find whether "users must enter a user name and password to use this computer")

Either way, as long as you've got a second user login, with admin rights and passworded, on the computer, it will help you recover problems should they arise.

Hope this helps.