Hidden in the bowels of Windows is a super-secret Administrator account. Here’s how to unlock it, in case you ever need to use it.

Deep inside Windows, there’s a secret Administrator account, and it’s different from the normal administrator account you most likely have set up on your PC. Oddly enough, this Administrator  account is not part of the Administrator group. (Note the differentiation between the secret Administrator account, and the administrator account you’ve set up. In describing this hack, we’ll always use the capital “A” for the secret account, and a lowercase “a” for an administrator account you’ve set up.)
What’s the difference between the secret Administrator account and a normal administrator account? On Vista, the difference is more than the name: the Administrator account is not subject to User Account Control. So the Administrator can make any changes to the system and will see no UAC prompts.For this reason, you may want to unlock the Administrator account, and use it only for those times when you want to make a series of system changes and don’t want to be bothered by UAC. True, you could instead simply disable UAC on your system, but it’s a pain to do this, and you may forget to turn it back on.

Turning on the Administrator account is pretty straightforward. On Vista, open an elevated command prompt by typing cmd.exe into the Search box on the Start menu and pressing Ctrl-Shift-Enter. Next, enter this command:

net user administrator /active:yes

On XP Professional, fire up TweakUI [Download from Microsoft website ] , go to the Logon section, and choose “Show Administrator on Welcome Screen”.
On Windows XP Home, the Administrator account is only available when you boot into safe mode.
From now on, the Administrator account will appear on the Welcome screen. Use it like any other account. Be aware that it won’t have a password, so it’s a good idea to set a password for it by going to Control Panel→User Accounts and Family Safety (Vista) or Control Panel→User Accounts (XP).
If you want to disable the account and hide it, enter this command at an elevated command prompt:

net user administrator /active:no