There are a couple of ways:
Open a command prompt (Run as Administrator.)
Type the following command and enter.
net user administrator /active
Log out and see if you have a new alternative login, as well as your existing one. If not, do it the long way.
Shut down the computer for a cold boot. Tap the "F8" key as you are booting.
Select "Safe Mode with networking" from the boot menu.
Log into windows 7 with your personal account that holds the administrator access.
Open a command window (START--->RUN--->CMD.exe). At the command prompt type the following
net user administrator /active
Log out and log back in as administrator.
Another way
Go to Start
Type Control UserPasswords2.
Click Advanced.
Click Advanced again.
Select Users.
Select Administrator and untick the the box “Administrator is disabled”
Now log out and login as Administrator.
It is hackneyed that the action leaves you a little more vulnerable to outside attack. Not a big issue if you are confident with your anti virus control etc.
Any hacker who can get onto your computer, merely because you are the Administrator, certainly knows how to get the same result through a user account. Root kits can be possibly planted more easily, but the best protection for those is a good firewall and hope.