Windows Vista contains both the familiar Internet Protocol v4 stack and the new IPv6 stack.
IPv6, when it's rolled out across the Internet over the next few years, will bring an end to the problem of running out of IP addresses and improve other aspects of TCP/IP operation.
However, if you are using Windows Vista on a network with Windows XP systems, you may have problems accessing password-protected shares on a Vista system from an XP system because Vista uses IPv6 and IPv4, while Windows XP uses IPv4 only.
The solution: Disable IPv6 on the Windows Vista system.
To disable IPv6 on a specific connection, you can go to the Network Connections folder, obtain properties of the connection and clear the check box next to the Internet Protocol version 6 (TCP/IPv6) component in the list.
This method disables IPv6 on your LAN interfaces and connections