Windows 7 system has a default administrator account that is created during the installation of Windows 7 or setup already for you on a OEM (ex: Dell) computer. By default, the built-in administrator account is named Administrator, and is disabled. Log as that account you will have complete access to the computer and can make any desired changes and will not be prompted by UAC.
This will show you how to enable Windows 7 default administrator account
Warning: if you enable the built-in Administrator account, it is recommended to create a password for it to help keep it secured.
1. Start System and press "F8" to enter Safe mode with Command Prompt
2. Choose any Administrator account and you will enter the elevated command prompt, type net user administrator /active: yes and press Enter.
3. Close the command prompt. Restart the computer and you will now see the built-in Administrator account log on icon added on Screen.
If you want to disable the built-in administrator account, just type net user Administrator /active: no, then you will see the built-in Administrator account log on icon removed.
1. Click Start and type lusrmgr.msc in the search box, open the Local Users and Groups manager, in the left pane, click on the Users folder.
2. In the middle pane, right click on Administrator and click on Properties.
3. Uncheck the "Account is disable" and click Apply. (if you want to disable the account, just check the" Account is enable".
4. Restart your system; you'll see the default administrator account appear on the log in screen.
Note: if you lost windows 7 default administrator password, you can download Windows Password Key to remove the password, it is the safest and more quickly way.