Checkout the <a href=”http://home.eunet.no/pnordahl/ntpasswd/”>Offline NT Password & Registry Editor</a>. Even though it says NT, it seems to work well on all desktop systems. Try on a test system first to ensure that you will not damage the system of interest.
Is this pc part of a domain ?
If so, you could log on with any account part of the “domain admins” group, to enable the local Administrator account.