If for whatever reason you must login Windows Vista with the Administrator account this guide will show you how it’s done.
Note: If you’re trying to stop User Account Control (UAC) popups when you change settings or install software you should try disabling User Account Control  before logging in as Administrator.
Warning: Running as Administrator in Windows Vista bypasses all security (UAC) and is NOT recommended. If you decide to use the Administrator account don’t complain when you start having problems.
Learn how to log in Windows 7 as Administrator  too.
Enable the Administrator Account
- Open the command prompt with Administrative privileges by opening the Start Menu, and typing cmd in the search box, and then press Ctrl+Shift+Enter or click the Start orb, All Programs, Accessories, right-click Command Prompt and select Run as administrator.
- Type the following in the command prompt and press Enter after:
net user administrator /active:yes
- Restart your computer and logon as Administrator.
Note: You might want to set a password for the administrator’s account for at least a little protection .
Disable the Administrative Account
To disable the Administrative account run the Net User command demonstrated above while logged on an account with administrative privileges but not as the Administrator account and replace yes with no.