Vista - Networking question.

I have two computers that I would like to be able to connect to one another if possible. One is being used as a music and video server with my Russound Cav6.6 whole house audio video system. The other is in the office.
The computer in the a/v room is running XP Pro SP3 and the office is Vista Ultimate 64bit. I am connected through a 2wire router and am hard wired and not wireless. What I want to be able to do is access music and videos off the office computer. I would also like to control and work with a program called Casa Tunes that the A/V computer has on it. The problem that I am having right now is that it recognizes the computers on the network but it won't let me in. I have been able to connect before but I can't seem to now. Is there something I might be missing? Thanks.