How do you find out someone's password?
You can remove the admin password of your locked computer then you can logon as administrator.
A useful tool just like Windows Password Rescuer can perfectly solve this problem.You also can use it to create a new administrator account without login then you can logon directly with the new account.
If your system is Windows 7/vista/xp,you can use "Windows Password Rescuer Personal/Professional'.
If your system is Windows 2008/2003/2000,you can choose "Windows Password Rescuer Advanced".