You can do this a number of ways but the best way since you mentioned it would be to establish a domain on the network in question.
1. On the Windows 2003 Server you need to run a command named dcpromo to promote your server to a domain controller. Follow the on-screen instructions.
2. You can continue to use the IP addresses assigned by your router or you can let the Windows server function as the DHCP server and issue addresses. This is up to you. If you have a typical consumer DSL router I would recommend you to let Windows Server 2003 perform the DHCP functions because a lot of DSL router's DHCP do not support proper name resolutions for the domain functions when you let it issue the IP address information. So you could run into situations where you try to access a server by its system name and you won't get any connection because the DSL router is not aware of the system's name. At this time you should also establish a DNS server role for the sever.
3. Configure the DHCP server to issue the same IP range that your current DSL router is using and disable the DHCP functions on the router.
4. After you got the domain controller up and going you need to create user accounts for each user. You also at this point will want to enable the File and Print services feature of Windows 2003 server. You can create a file share and give ALL users that you previously created read, write, or read and write permissions to this share.
5. Next steps would be connecting to each individual PC and joining them to the domain. The systems in question in the case of Windows XP need to be the Professional edition. Windows XP home will not work. In the case of Vista it needs to be Vista Business,Ultimate, or enterprise - vista home and home premium will not work. In the case of Windows 7 it needs to be Windows 7 Professional, Ultimate or Enterprise - starter and home editions will not work. In all cases you can join the system to the domain from the Control Panel --> System.
6.
After you have done all of this you need to add the shared folder as a shared drive on each system. You can do this manually or the better way is via group policy which is a bit more complex to describe all in detail here. Basically group policy allows you to set settings on a large subset of systems via a script that will run whenever a domain user logins or whenever a domain joined system connects to the network.
To do it manually you could type at a command prompt: net use [drive letter here]: \\servername\filesharename
So for example net use z: \\mydc\mysharename would create a drive letter of Z: on the system that is connected to a server named MyDC that has a fileshare name of mysharename. The user in question needs to have the appropriate permissions on this share otherwise the connection attempt will fail.
7.
Now you said you want to have each users documents and email available on every computer they login to I'm assuming is what you are trying to say. In this case you will need to use a roaming profile which will cause the users settings to be stored centrally on the server and will 'roam' to the system that they login into when they connect.
This will require an additional file share that is unique to each user.
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