Using VPN To RDC With Windows 7

I'm trying to get a client access to their desktop at the office so they can do pay-role and a few other things while they are at home. The old computers that they had were XP pro and it worked fine for them, the server itself is a win2k server. Recently installed all new computers, minus the server, with Windows 7 pro and obviously now they cant use the RDC.The server doesn't seem to be setup securely and they are using a Cisco 1720 router that I cant get access into, though I don't think you can anyway. every computer on the network shows the same IP when connected to whatismyip.com, so I'm not entirely sure how to get them directly to the computer they need.There is a wireless, but only for the Laptops and is a simple AP anyway. When I try to connect using the IP, I get prompted for a user/pass but it doesn't seem to want to accept the server's user/pass, nor the actual computer's user/pass.