Vista uses a new protocol named Link Layer Topology Discovery (LLTD) to display a network “map” of all computers in a network, but the protocol is only in Vista, so XP computers do not show up in this map.
Microsoft has generously released the software for XP, and it must be installed on an XP machine for it to show up in the Vista map. Click here to download the software for Windows XP SP2.
Source : Top 12 Vista Tweaks
