"One of your NetBIOS names is already registered on the remote network.."

This error usually occurs when you are dialing up to your ISP.

In your connectoid's properties make sure that "Log on to Network" is NOT checked.

In Windows NT4.0, this error could be caused because a computer on the network already has the same name as your computer. Check the name of your computer in windows networking, and if necessary change the name of your computer.

Also this doesn't happen often, but make sure that the computer you are using isn't already connected to the network you are trying to connect to.. This will cause the same error (and there should be no need to connect your machine to the same network twice anyway)

This is a very similar error to Error 634 - Cannot register your computer on the remote network.