1. If you are wanting to run a file/print server in a corporate environment then I highly suggest using a Server OS. I believe there is a 10 user limit with XP and that could hinder you in the future if you don't already have 10 people that would be accessing the file server.
3. You cannot setup a Domain on XP. You need to have a Server OS to setup a domain. You can add a XP machine to a Domain but you will need a server to be the Domain Controller.
When you set up Server 2003 you have to run dcpromo to make it a Domain Controller. This promotes the machine and allows you to set up a Domain for your network. Then you use Active Directory to manage that Domain by using user and group permissions and policies.
To be honest it is not going to be an easy task at all if you know nothing of how to set one up. There can be a lot of troubleshooting involved and unless you understand how a network works and how the Server works you are going to be ripping your hair out.
My suggestion is to get a book and study up on it. If you get a kit like THIS
then it will walk you through how to set all of this stuff up. I also highly recommend THIS
book, as it is a little bit easier to understand if you are new to this.
If you are going to be the Sys Admin for your company then studying and learning as much as you can on networking and Server 2003 (since that is what you are using), because you will need it. Granted your network isn't all that intricate, but you need to have a foundation of knowledge first before you can start to build from it.
4. I apologize I am used to using that acronym. You can still set up a similar situation in Server 2003. You just set it up as a Secondary Domain Controller.
Good luck to you.