Vista pc hidden from XP?

"Vista and XP networks, in my experience, are quite incompatible. Interaction might work, or it might not. The network might work correctly today, but not work tomorrow.

The major problem is defining the master browser computer for the network. The article Computer Browser contains a good explanation of the problem, and how to assure that the right computer is elected as the master. If not enough, other articles are easy to google.

The problem to avoid is for your Vista machine to come up and decide that it's the master browser of its own network of one, while the XP computers elect one of themselves as master browser of their own network.

To make the XP machines discoverable by Vista, you should install on them the Link Layer Topology Discovery (LLTD) Responder.

You should think the problem through quite carefully, decide on the correct network architecture and define the right master browser (which you might need to enforce), and try very patiently to make it work correctly.

If everything fails, the last resort is giving all the machines fixed IP addresses and updating the hosts files on your network to contain all the addresses.

EDIT

Check this article : File and Printer Sharing in Windows Vista.

Other points:

Workgroup Name of all the computers should be the same.
In Vista Network and Sharing:
Network Discovery: ON (So it can see the other computers)
Network set to Private (Public is for hotspots, airports, etc)
File Sharing: ON
Public Folder Sharing: ON (Vista Public Folder is the same as XP Shared Docs)
Password Protected: OFF (unless you want to set up identical usernames and
passwords on all computers in your Network). If you have it ON, you will be
asked for a username and password when you try to access a Vista computer
from an XP computer.
Run the XP Home Network File and Printer sharing Wizard."

"Despite what the other user said, Vista and XP are fully compatible on a network. You just have to fiddle with the settings to get everything shared out properly.

Since you can ping the Vista box by both IP and UNC we know that it's live on the network. What you're running into is something on the Vista PC blocking access to the files and folders. I'd start by checking the network settings on your Vista PC. Try either disabling the firewall or setting the network to Private instead of Public. For more information, try this page from Microsoft. Good luck!"