Windows 7 and Vista have the ability to detect the devices on a network and build a network map based on what it finds. Windows 7 and Vista do this using a protocol called LLTD (Link Layer Topology Discovery). The LLTD Responder is built in to both Windows 7 and Vista. It's also being included in Access Points, Wireless Routers and some NAS devices that display the "Windows 7" Logo.
Among other things LLTD does is detect laptops and desktops that are connected via wireless and builds the network map accordingly. However, any wireless connections will show up only if there's something connected.
However, XP doesn't support the LLTD Responder natively and thus, when the network map is displayed, any PC's running XP (any flavor) will show up on the bottom of the page in their own area, unable to be placed on the map.
Microsoft has a hotfix that will add LLTD Responder support to XP. It does require a Genuine copy of XP and may also require providing your email address to get access to the hotfix. The hotfix only works with SP3 so make sure your system has it installed.
IMPORTANT: This hotfix needs to be installed on every PC running XP on the network.
Here's the link:
If you look down for the section marked "Resolution", you'll find the link to the hotfix.
To get the hotfix, it's best to use Internet Explorer as you will need to go through Windows Genuine Advantage Validation.
Once it's downloaded, install it on any PC's that are running XP. You may need to reboot the PC afterwards.
Once it's installed, try looking at the Windows 7 Network Map and your XP machines should now display in their proper place in the map.