How can I by pass the login box on a pc?

I am actively completing my internship at my current employer ( Alcoa Fujikura Ltd.) and I am the IT Assistant. I am inventoring and auditing all the pc’s in the facilities as part of my internship. We do not have a official IT Personnel, besides our main branch id SC, and need help accessing some pc’s. Most of these pc’s have network log on prompts and require a user and password. Well the real problem is that since the layoff we have extra pc’s that need servicing and maintainance. Only the pc’s keep prompting me for a valid domain and user/password that no longer exist. How can I by pass the system log on? FYI: they all run Win2000Pro.