You log off of your current user and then log on as Administrator.  It is disabled by default, so you will have to enable the account and set a password before it will work (Right-click Computer, Manage, Users/Group).  If you have never used the account, it is not necessary.  What are you trying to do?  
HOW DOES ONE LOG ONTO VISTA AS THE ADMINISTRATOR?  HELP PLEASE! ronquart:emotion-55:
Windows Vista 