Administrator Account

Local (Hidden, Built-In) Administrator Account in Windows 7
1. First you'll need to open a command prompt in administrator mode by right-clicking and choosing "Run as administrator" (or use the Ctrl+Shift+Enter shortcut from the search box)
Run Command Prompt as Administrator

2. After that you only need to enter the simple command below to activate it.

net user administrator /active:yes
enable hidden administrator account

3. You should see a message that the command completed successfully. Log out, and you'll now see the Administrator account as a choice.
built-in windows 7 administrator account

You'll note that there's no password for this account, so if you want to leave it enabled you should change the password.

More can be found at this MS link