Reset lost password of Windows 2008 R2

The process is simple:

1) Boot Ubuntu 12.04
2) Open the Disk Utility and mount the partition that contains the Windows OS on it (make note of the partition name).  Let us call this “PartName” for now.
3) Open the Terminal application and cd to /media/PartName (where PartName is the name of the Windows partition).
4) cd to Windows/System32      (you are in Ubuntu, so case matters, check the case)
5) cp Utilman.exe Utilman.exe.bak
6) cp cmd.exe Utilman.exe
7) Power off, remove your Ubuntu disk and boot Windows.
8) At the login prompt, press <windows-key>+U
(this will bring up a cmd.exe window running with administrator level privileges)
9) net user <username> <new password> (eg. net user Administrator Pwned123!)

You have just the password for the Administrator account.  Type ‘exit’ to exit the command prompt, and since you are at the login screen, feel free to press CTRL+ALT+DEL and log in.

Don’t forget to open a command prompt window, navigate to c:\windows\system32, and copy Utilman.exe.bak Utilman.exe.  Otherwise, you have left the server open to anyone who walks up and presses <windows-key>+U.  Although it still won’t stop them from doing what I did.

Source: https://joeweaverwindgap.wordpress.com/2013/08/04/how-to-reset-windows-server-2008-r2-domain-controllerdomain-administrator-account-password/