Do you mean that the computer can only log on as part of a domain? like you are being blocked at the login screen, and cant login using it as a workstation?
Or, do you mean how to reset the way the computer connects to the internet, or how it identifies itself with a network?
If you can login fine, and just need to change the network settings for the computer, you can do that easily from the 'System Properties' menu. It can be found in Control Panel -> System, or just hit WindowsKey+Pause, and it will come up.
Those options are under the 'Computer Name' tab. You will need admin priviledges to do that.
If you cannot login to the computer, that means it requires a domain login to access it. You will need to reinstall. If you can access a user interface in safe mode, I recommend running a program called ' Aida32 .'
It will show a very in-depth version of the system info, and from that, you can learn the key used to install windows on that machine.