You cannot accomplish what you are trying to do without a domain controller, XP cannot act as a domain controller, you need NT, server 2k or server 2003 for a windows based PDC....
Alternately, you could set up a linux or unix box, and run a samba server and use that as a PDC, then all your XP machines would authenticate against that and your user level security settings for any user logged into the domain would then show up in your security settings dialog..
Linux Unix solution is open source so it doesn't cost anything.... The downside is that it isn't point and click, but if you know anyone familiar with linux, maybe they could set it up for you. You could pick up an old pII for around $50 and it would run linux or unix just fine.
It wouldn't affect your ISP either, even though the external IP address of your modem is part of your ISPs domain, to your ISP, your internal domain doesn't exist.
