How to reset Windows 2008 R2 Domain Controller "Administrator" password?

Hello Everyone,

I have lost Administrator password for the following system:

Windows 2008 R2

Domain Controller setup on same machine

Stand alone server - no workstations or other servers invovled

I still have the "Directory Service Restore Password" but I don't think that helps me for lost Administrator password. I beleive I need to boot from an .iso file to gain access. I already tried "Offline NT Password & Registery Editor" and it has set Administrator password to (blank) but that is not allowing me access as it seems that I have to login to domain controller Administrator. So, how can I reset that password?

Thanks