Is the computer part of a network (domain) with Administrators? If so the administrator should be able to go in a reset his password.
If not and if there are no backdoor accounts on the machine, then there are boot disks that can be used to reset the Administrator password on the local machine and regain access.
I recall a boot CD called ophcrack, which was free, that would run and display all the passwords. I remember using it 3 or 4 times and it worked every time. If I were more ambitious I would look for a link, but I haven't had coffee yet.