First, this is not specific to WinXP. Anyone can install a NetBIOS capable OS and use your domain in the name field. Windows, Linux (using Samba) will all do this. Since they are all in the same NetBIOS browsing domain they will show in your Server Manager list, browse list. However, you will not be able to manage them, and they will not be able to manage your systems. At the time of installation, the various WindowsNT/XP systems install create a unique identifier, I think it is called a SID (Security IDentifier), which is a quasi-random, big number that is unique for all installations. When the OS is installed into a domain and you enter the domains administrator password, the SID derives part of itself from the domain controller. Alternatively you can admit the machine to domain later using server manager, when then stores the SID in the domain database. Otherwise, welcome to the joys of NetBIOS networking. Can't control it, can't lock it down. If you want to control these type of happenings then you should look at an Active Directory Services implementation which will control this. Feel free to ask more questions on this one.
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