Client to VPN to Server

I have a Windows 2003 server and LAN with 7 computers tied in through a HP Switch. Internet connection is through a Linksys VPN box. We just added an out of town office with 4 Windows 2000 computers. These computers are connected to the VPN. They have been given an account in the server. But I cannot figure how to connect them properly. I have mapped a drive so they have access to some files. I need them to be able to log on to the Server as anyone else on the LAN. If they open Windows Explorer, they need to be able to see the choices on the server. Right now “My Network Places” shows nothing on the LAN or Server to the out of town office. If they try to sign on under the domain, the domain does not exist.