Not many people know that Vista has a hidden super user account.  I will show you how to activate this Vista Administrator account via a 'Net User' command.  One benefit of logging on as this super account is that you will never be prompted for the nagging UAC dialog box.

How to Activate the Hidden Vista Administrator

Key decision - decide if you need a password for the administrator's account that you are going to activate.  My point is that the local policy may insist on a complex password, thus you will not be able to activate the administrator with a blank password.

Overview

1. Logon to Vista using your usual account.
   
2. Launch the cmd prompt - Make sure you select, 'Run as administrator'
   
3. Net user administrator p£ssw0rD
   
4. Net user administrator activate:yes
   
5. Switch User, or logoff
   
6. Logon as Administrator  Password p£ssw0rD 
   (Your password may be different!)
   

Detailed Instructions to Activate the Administrator

1. Logon to Vista using your normal username and password.
   
2. Click on the Start button
   
3. Click on Start Search. 
   
4. Type, cmd. 
   
5. Right-click cmd, select 'Run as administrator' from the shortcut menu.
   
6. In the black 'DOS box', type the following at the command line:
   Net help User
   
7. The idea of the last command is just to observe the options for Net User.  In particular examine the syntax to set the password.
   
8. The next instruction is the crucial command.  I have chosen password = p£ssworD, you may want to choose different characters.
   
9. Net user administrator p£ssw0rd
   
10. Net user administrator activate:yes
    
11. Check the message : The command completed successfully
    
12. Switch User, or logoff
    
13. Logon as Administrator  Password p£ssw0rD  (Your password may be different!)
    

Trap:  There should be no space between the word 'activate' and the colon.

        Activate  :yes is wrong.   activate:yes  is correct.

Local Security Method to Enable the Administrator Account

If you prefer a simpler method, visit the Local Security policy, incidentally, this MMC is worth exploring in its own right.

This is how you navigate to the Local Security Policy. 
Firstly click on Vista's Start orb, then in the Start Search dialog box type: secpol.msc.  Note: you must include the .msc extension.

Secondly drill down to Local Policy, Security Options

Thirdly double click Accounts: Administrator account status, and select enable.