Windows Vista and Windows 7 employ similar security mechanism to that used in Ubuntu Linux: the full priviliges administrator account (root in linux) is by default disabled, and it is not possible to log on to it. Unfortunately sometimes an admin level access is necessary, for example to restore a broken user profile.
To activate the Administrator account, right click the Command Line from Accessories menu and choose Run as Administrator:
Type the following:
net user administrator /active:yes
When you no longer need this extra account you can disable it:
net user administrator /active:no