You said you can't access the server, is this a local admin account or a domain admin account?
If domain you can give this a try
http://www.petri.co.il/reset_domain_admin_password_in_windows_server_2003 _ad.htm
If local than id10t suggestion should work.
Like Halm said there should be a backdoor, another admin account should have been created. Do they have any documentation on the members of domain groups, enterprise admins, domain admins, etc? Also if they have a tape backup account it may have admin rights as well.
Of course before I tried anything I would make sure they have a recent backup.
good luck and let us know how it goes.