Try Safe Mode and see if you can enter the Administrator account with a blank password.
This article is not as hopeful as it sounds but have a read anyway.
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There are many sites that can help you further with lost or forgotten passwords (Google for instance) but this site for security and ethical reasons does not support that policy. Sorry about that, but if we can help in any other way please try us again.
Recovering a user's password. (Help and Support)
If you forget your password on a computer that is part of a workgroup or is a stand-alone computer, check your password hint. (Your password hint is located on the Welcome screen beside the password prompt.)
If you have a password reset disk, you can use it to create a new password for your account. Your personalized computer settings will be retained. For more information about using the Forgotten Password Wizard to create a password reset disk, click Related Topics.
If your password hint does not help you remember your password and you did not create a password reset disk, you will need to have someone with a computer administrator account create a new password for you. For security reasons, a computer administrator cannot recover your password if you forget it; he or she can only create a new one for you.
Note
A user with a computer administrator account can create or change passwords for all users on the computer. Users with limited accounts can create and change their own passwords, as well as create password hints.
Big Note**************
If a user with a computer administrator account changes the password for another user, that user will lose all EFS-encrypted files, personal certificates, and stored passwords for Web sites or network resources.