It's called setting up a domain.
It requires that you purchase and set up a domain server, with the appropriate software. Then set up all workstations to authenticate from the server.
If you purchase Windows Server to provide these services, be prepared to buy seats as well, based on the number of users.
You can do this for far fewer dollars using Linux, but the learning curve can be a bit on the steep side.
Some Windows OS distributions, such as XP home edition, do not support domains, so that may or may not be an issue, depending on what you are running
All in all, not for the faint of heart.
If you are not familiar with the issues with domains, and with domain administration, I would bring in a contractor to set you up.
Good Luck.