Windows Vista’s new Network Map feature (accessible from the top of the Network and Sharing Center) gives you a visual of the basic layout of your network. The detection of devices is made possible by the Link Layer Topology Discovery (LLTD) Responder protocol, which is integrated into Vista. Windows XP however doesn’t include this protocol by default; you must download and install it on XP machines, if you want them to appear on the Network Map.
You can either ensure the appropriate Hotfix for Windows XP Service Pack 3 is installed or you can manually download and install the LLTD protocol from Microsoft’s site.
Eric Geier is an author of many wireless networking and computing books including Home Networking All-in-One Desk Reference For Dummies (Wiley 2008) and 100 Things You Need to Know about Upgrading to Windows Vista (Que 2007).