I've written about LLTD in the past but the XP support files weren't publically available previously. With Vista RTM'd, MS has released the XP files needed to "show up" properly on the network maps.
They are better than writing a description than I can, so here is how they describe LLTD:
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.
Go
here to add support to XP for LLTD.
