During the Windows 7 installation create a useraccount, let say USER1. This user is an "administrator" by default now.
You will call your computer PC-NAME
After installation keep the pc in a workgroup (so do not add the computer to a domain)
Now log on to your computer as: PC-NAME\USER1
Now create a new account: USER2 and do not add a password to this user
USER2 will be a regular, normal user, without administrator right or any other special permissions.
Now, add your computer PC-NAME to a domain. You will add your computer PC-NAME to (for example) the domain ABC.
Now restart your computer.
Now your pc is a member of the domain ABC.
At this point you will log in your computer locally as: PC-NAME\USER1
When you are logged in as PC-NAME\USER1 you have administrator rights to disable your current account.
So you will disable PC-NAME\USER1 and you will see that by default PC-NAME\Administrator is also already disabled.
So when both PC-NAME\USER1 and PC-NAME\Administrator are disabled, you will restart your computer.
When your computer is back on you are asked to log in again.
Try to log in as a local user with administrator rights! For as far as I know, you have a computer without a accessable administrator account.
Can you get the administrator enabled again?