normally try
net users administrator /active:yes
if you run windows 7 french version try
net users administrateur /active:yes
There are two problems with this feature of the operating system.
First, by default, the administrator account does not show up in the list of available accounts when Windows 7 boots up. This is why so many people do not know that the account even exists.
Second, there is no password set for the administrator account. These two issues combine to create a moderate to major security risk depending on how you use Windows.
http://helpdeskgeek.com/windows-7/secure-the-windows-7-administrator-account/