In order to logon as an Administrator you need to perform the following actions. These will work for both WinVista and Win7.

1. Enable Built-in Administrator Account
   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)
   
   
2. 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.
   
   
3. 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