With the server running as a stand alone server, the ONLY user that will be allowed to log on locally is the Administrator account, unless a user has been given specifically the permission to log on locally (which is what "interactive" means), and has a local user account. Servers, by default, deny local access to all users except the Administrator. Being in the administrators group on this system still means you need a local user account, and you must have local access turned on before you can log on locally.
Log on as the Administrator account, go to the control panel, administrative tools, and select local security policy. When the local security settings window comes up, select local policies, User rights assignment, and make sure the user account you are trying to use is listed as being allowed to log on locally. While you are there, set the permissions on your local account for local access and any other things your account will need to do.
If you can not access with the Administrator account, or can not remember the password used when the system was built (try a blank just for kicks!), then you will need to re-install the OS and make sure you remember the administrator's password for future access!
Remote Users should be able to access the server from the network, but since this server is not in a Domain, the remote user MUST first have a local users account set up in order to be allowed access.
HTH,
David