There are one or two established ways. You can find most of them by searching (recommended) this site. But for your convenience.
1. 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.
You might also find the attched files useful. Run the appropriate .reg and it will automatiically give you the right click option on the mentioned files.
2. Another way
Go to Start
Type Control UserPasswords2.
Click Advanced.
Click Advanced again.
Select Users.
Select Administrator and untick the the box “Administrator is disabledâ€Â
Now log out and login as Administrator.