Normally it doesn’t possible specially in Windows 7. The best way to use password reset disk to reset the password.
U may disable Administrator Account through Local Users and Groups
NOTE: This option will only be available in the Windows 7 Professional, Ultimate, and Enterprise editions.
1. Open the Local Users and Groups manager.
2. In the left pane, click on the Users folder.
3. In the middle pane, right click on Administrator and click on Properties.
4. To Enable the Buit-in Administrator Account
and Uncheck the Account is disabled box.
5. To Disable the Buit-in Administrator Account
A) Check the Account is disabled box.
Now click OK
Close the Local Users and Groups window.
Now log off, and u will now see the built-in Administrator account log on icon added (enabled) or removed (disabled) from the log on screen.