iacy

There is unfortunately no simple answer to this problem based on my
experience with small business and home networks from a user viewpoint.
You are lucky if it is only a firewall issue. In my experience this is
rarely the problem as most people use the Windows firewall and have used the
network wizard to set up printer and file sharing and have properly allowed
sharing on their individual drives or folders and proper exceptions in the
Windows firewall. Something went wrong in the automated process which is
Microsoft's problem but they foist it off on the end-user.
Rarely will shutting off the firewall or anti-virus program clear the
problem but it is worth trying as a troubleshooting measure.
The problem is usually on the computer that can be seen on the network but
will not allow its files to be shared. Usually it cannot open files from
other computers on the network either but can access the Internet.
This happens on all XP networks, not just XP/Vista mixed networks and is
inherent to the way these versions of Windows can screw up how they
implement network protocols. Microsoft pretends this does not happen or
there are simple ways to straighten it out but nothing could be further from
the truth.
In all honesty I have no clue how the TCP/IP drivers allow you to use the
network to access the Internet (e.g. through your DSL/Cable modem-router)
but not other computers on the network.
The Microsoft Knowledge base can walk you through ways to supposedly reset
the TCP/IP stack through some simple command line orders. You are lucky if
this works.
You can uninstall the protocols, uninstall the drivers for your network
card, reboot and reinstall all and redo the network wizard. If you are lucky
this will work.
However often it does not.
Suggestions beyond that include trying to use NetBios (good luck on that!)
or reinstalling Windows.