How to unlock computer without knowing current password

This trick based on the substitution of the Utility Manager (Utilman.exe) program that can be used during logon screen with Command Prompt (cmd.exe), so you can access Command Prompt from the login screen and reset your Windows 7 password quickly. Follow these steps:
Boot your computer from Windows 7 installation disk. When the Install Windows screen appears, press SHIFT + F10 keys to open the Command Prompt.

Type in the following commands. Replace C: with the drive where your Windows 7 system was installed.

C:
cd windows\system32
ren Utilman.exe Utilman.exe.bak
copy cmd.exe Utilman.exe

That's all we need to do here. Close the Command Prompt and the Install Windows dialog to restart you computer.

Let the Windows 7 boots normally this time. At login screen click the Ease of Access icon, it will launch the Command Prompt window. You can then run the following command to reset your forgotten password.
net user username new_password

Close the Command Prompt and you can use the new password to log in. Don't forget to restore the original Utilman.exe file back.

If you're still unable to reset Windows 7 password with Command Prompt or you have no prior knowledge of the Command Prompt