Authenticate across a WAN

I have 2 needs. One is to have one user log onto a domain remotely over a WAN link. The user is assigned a valid IP. What I want is for him to start up and log on, using the domain’s resources. not the local ISP’s. In other words he would log on and go back out to the Internet using the domain’s resources not his local ISPs
#2. How can I get a DC to authenticate to another DC across a WAN? Both are going to be part of one domain. All DCs are W3K server and workstations are XPpro