How to Reset Your Forgotten Domain Admin Password on Server 2008 R2
Forgetting your password is always a pain, but luckily there’s an easy way to reset your Domain Administrator password. All you need is a copy of the Windows Server 2008 R2 installation disk and one simple command line trick.
Replacing Utilman.exe
Boot off the Windows disk and select the “Repair your computer” option from the lower left-hand corner.
Follow through until you get to the option to open the Command Prompt, which you’ll want to select.
First you’ll want to type in the following command to backup the utilman.exe file:
MOVE C:WindowsSystem32Utilman.exe C:WindowsSystem32Utilman.exe.bak
Now you will need to copy cmd.exe and rename it Utilman.exe:
COPY C:WindowsSystem32cmd.exe C:WindowsSystem32Utilman.exe
Now you can go ahead and reboot your machine. When its done booting up again and you are at the Logon screen click on the Ease of access icon.
I bet you weren’t expecting that open a command prompt 
To change the password type:
net user administrator *
Once you press enter you will asked to set a new password and then confirm it, when entering your new password don’t worry if you can’t see them as you type, they are invisible, they are however being remembered.
Once you’re logged in again don’t forget to delete Utilman.exe and then rename Utilman.exe.bak back to plain old Utilman.exe.