Remote Desktop Help

Here is what I have – 2 Office Computer’s behind a router (XP pro and Vista), I am trying to access the Vista machine remotely however I can only pull up the XP Pro machine. I am not sure how to pull up my Vista machine and I have tried using the IP address but it isn’t working. I am using the IP address that the router has given the computer. Is there something I need to change in the router or on the computer? The router that I am using is a Netopia. Please help.