How to unlock 2000 Pro

I have a simple peer to peer network at my home that I run with 2000 Pro. Recently I ran one of those treak utilities on one of the clients. When I was done running the program, I did a restart and now somehow the username which was left at default is now gone. It will not except default as the username. If you have the trick to getting around the username/password, I would appreciate your assistance.
Thank you.