Windows NT User Question

Hello, well recently one of my neighbors has bought a building where we found a computer. It's running windows NT. However, I cannot login. It will not let me login as guest because it is disabled. I do not know the passwords for the usernames. I tried running a program that supposily changes the password, but it is not working. The program that I am running has a registry editor built in. I was wondering if there was a way that i could add a user to the registry ( without a password of course ) I don't know if there is some kind of registry value that i could put in so that i can login. I do not want to reformat the hard drive. I know that there is a way to some how login. Also, please be aware that I do not have physical access to the computer. ( Just through the password changer program )

Thanks