A Windows Vista computer cannot see a Windows XP System on the network
Fix: Download Link Layer Topology Discovery (LLTD) Responder (KB922120)
File name: WindowsXP-KB922120-v5-x86-ENU.exe
File Overview:
This package contains the Link Layer Topology Discovery (LLTD) responder. The network map on a computer running Windows Vista shows a graphical view of the computers and devices on your network and how they are connected by using the LLTD protocol. The LLTD responder must be installed on a computer running Windows XP before it can be detected and appear on the network map. After you install this item, you may have to restart your computer.