Windows 7 provides a default administrator account primarily used for troubleshooting and testing purposes. Users with administrative privileges have the same capabilities as the default administrator account. However, it is ideal to use the default account if you do not have enough permissions in your existing profile and only need to troubleshoot your computer as a one-time event.
This effectively prevents you (or an active administrator) from elevating your current account's access, so it is also recommended that you check your account type prior to using Windows 7's default administrator account.
Check Account Type: Domain
Log in to your account from the Windows 7 main Welcome screen. Click Start | Control Panel | User Accounts | User Accounts | Manage User Accounts. Type your current password or provide confirmation if prompted by the User Account Control dialog.
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Note your account type under the Group column, shown on the same row as your Windows user name. If the account type is marked as "Administrator," your account already has administrative privileges, and you do not need to use the built-in, default Windows administrator account.
Check Account Type: Workgroup
Log in to your existing account from the Windows 7 Welcome screen. Click Start | Control Panel | User Accounts and Family Safety | User Accounts | Manage Another Account. Type your current password or provide confirmation if prompted by the User Account Control dialog.
Note your account type as shown under your user name. If you lack the necessary administrative privileges, you must use an account with such capabilities to use the default Administrator account or ask another administrator to provide you with said access. However, note that if your account is marked as "Administrator" you do not need to use the built-in, default account.
Sign in as Administrator
Click Start and type "CMD" into the built-in Search field to enable the Windows 7 default administrator account. Right-click "CMD" from the Programs group shown, then select "Run as Administrator." Type an Administrator password if you are launching this program from a non-administrator account. Otherwise, ask an active administrator to provide his password to proceed.
Type "net user administrator /active:yes" and press "Enter" to effectively enable the default Windows Administrator account. Close the window at hand.
Now go to the Welcome screen by clicking Start, highlighting the right arrow next to the Search field, then clicking "Log Off" or "Switch User." Click the Administrator account that now appears on the screen to log in. Note that this account does not have a password by default.
Tips for Default Administrator Account
Disable the default administrator account if it is no longer in use. Launch the Windows Command Prompt (CMD) once again, then type "net user administrator /active:no" and press "Enter."
Create a password for the default administrator account if you intend to leave it enabled. From the Windows Command Prompt, type "Net user administrator Password" where "Password" represents the actual password you wish to create. Press "Enter" to complete the procedure.
You cannot enable or disable the default Windows 7 administrator account if you do not have administrative privileges yourself (either on your own account or that of an active administrator).