When Microsoft released Vista they decided to disable the built in Administrator account. This continued to be the case in Windows 7.
90% of the time you should not need to use this account but there are the odd occasions when it is useful to have around.
To enable it you need to do the following:
Open a command prompt in administrator mode by right-clicking and choosing “Run as administrator”
Now type the following command:
net user administrator /active:yes
You should see a message that the command completed successfully. Log out, and you’ll now see the Administrator account as a choice.
You’ll note that there’s no password for this account, so if you want to leave it enabled you should change the password.
Disable Built-in Administrator Account
Make sure you are logged on as your regular user account, and then open an administrator mode command prompt as above. Type the following command:
net user administrator /active:no
The administrator account will now be disabled, and should not show up on the login screen anymore.