Reset Administrator passowrd of Win server 2012

? Boot from the Micrsoft Windows Server 2012 DVD

? From the Windows Setup menu,click “Next”.

? Select “Repair your computer”

? Under Choose and option,click on “Troubleshoot”.

? Under Advanced options,click “Command Prompt”.

? At the command prompt,run the following commands:
d:
cd windowssystem32
ren Utilman.exe Utilman.exe.old
copy cmd.exe Utilman.exe

(d: is the local windows drive,if can’t be sure,use command diskpart and list volume to show local system drive)

? Close the command prompt and then click “Continue”.

? The server should now boot and present the logon screen. Here click Windows Key + U. (if hotkey didn’t work,click the icon to start utilaman.exe which replaced by cmd.exe)

? utilman

? At the prompt you can now change the password,by typing the following command:
net user administrator Password123
This will set the password for the Administrator user to be Password123 (case sensitive).

Closing the command prompt,you should now be able to log back onto the server using the password you have provided in the last step.

Cleanup Steps

Once you have verified you can log on to the server you will have repeat the steps above and boot using the Windows Server 2008 DVD/ISO and run the command prompt again.

? Restart your server and once again,boot from the Micrsoft Windows Server 2012 DVD

? From the Windows Setup menu,click “Command Prompt”.

? At the command prompt,run the following commands:
d:
cd windowssystem32
ren utilman.exe utilman.exe.new
copy utilman.exe.old utilman.exe

? Close the command prompt and then click “Continue”.

You should be back up and running as if nothing ever happened.

KB Link:
http://www.kieranlane.com/2013/09/18/resetting-administrator-password-windows-2012/

Remark: How to find system drive letter

Typing the following command
diskpartlist volume(find system drive letter)then you can find system32and rename the Utilman and replace it with command prompt (cmd.exe)windows key + u = opens command prompt or click the icon.