In Vista SP1 the default Administrator account is disabled. But enabling it is a simple task.

Enable a command prompt with administrative privileges.

1. Enter "cmd" in the search box under the Start menu and press CTRL + SHIFT + Enter to launch the command prompt with administrative privileges.

or

2. Alternatively, you can also right-click the highlighted result and choose the "Run as administrator" option.

Now run the appropoiate command below.

To enable the account type the command:

net user administrator /active:yes

To disable the account, type the command:

net user administrator /active:no