If you do a search on the web for "recover domain admin password", you'll find a few methods/tools you can use/download to reset the admin password. You should note that none of them are supported by Microsoft so you are on your own.
If this is a production system, I would caution you. However, if this is a test system, give it a try. I have used some of these methods before and they work. There are just no guarantees that you wont blow up Active Directory in the process.