In Windows 7 and Windows Vista the local administrator account is disabled. Here are the following ways to enable the account. I recommend enabling this account if the computer will be part of a domain. You must be a logged in as a local administrator or know a local admin username and password.

Command Line
The nice thing about the command is that you can script enabling this account.
Select Start >Accessories > Right click on the Command Prompt > Run as Administrator
In the command prompt type the following command. This will enable the administrator account.
c:\> net user administrator /active:yes
Set a password for the administrator account. Replace the <password> with your own password.
c:\> net user administrator <password>

Computer Management
Select Start > Right Click on Computer > Manage
Computer management will load. Select Local Users and Groups > Users > Double Click on Administrator
Uncheck Account is disabled

Set the password in Computer Management
Right Click on the Administrator Account > Set Password