The new Network Map in Windows Vista allows you to easily create a map of all computers on your network and see how they are connected.  This works by using the Link Layer Topology Discovery (LLTD) protocol that is only part of Windows Vista. 

To help users of mixed environments of both Windows Vista and Windows XP, Microsoft has released the Link Layer Topology Discovery Responder for Windows XP.  This will make your Windows XP computers show up in the Network Map in Windows Vista.

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.

Click here to download the update for Windows XP SP2