Though I am sure this is not what you want to hear, if you forgot the administrator password and don't have another administrator account, you won't be able to reset the password. Since there are no other user accounts on the computer, you will not be able to log on to Windows and will need to re-install Windows.

Have you tried all the password possibilities that you can think of? If someone else created the password, check with them to see if they remember it.

Cheers,
Cassandra