Well thats easy!!!
All you need is the domain name, then the domain credentials as in the user name and password, and thats it!
Example......
You just install XP, time to join it to the domain. Go to system in the control panel, computer name tab, then click change to rename this computer or join to a domain, then select domain, type in the domain name, lets say its NA, just type that it, then press ok, a username and password prompt will appear, then type in na\genesys for the username and genesys for the password. Ofcourse this wont be your username and password for your network, but it is for mine. You may need to find the admin of the network to get this info. After you do that reboot and then add the local admin account which I add the name of the user as an admin, and thats it!
Does that make sense?