Home Networking Problem

I have 4 computers networked - three are XP and 1 is Vista. I have always been able to read/write files from and to all computers.

I just added a WIN 7 computer. I now have one problem - I cannot access the vista computer from the WIN 7 computer.

Everything else works fine. I can...
- access the WIN 7 computer from the vista computer and the XP computers
- access the vista computer from the XP computers (but not from the WIN 7!)
- access the XP computers from the Vista and WIN 7 computers

Any thoughts as to why this is happening?

On the new win 7 computer, I am using AVG (free version) and windows defender.

Thanks for your help,
Len