Something to try but no guarantee of success.
In Vista in Control Panel, Network and Sharing Center, you will see top right in small writing "View full Map"
The chances are if you check that it will show the XP machine at the bottom of the page. In other words not on the map. LLTD is required to create the map and it is installed by default in Vista.
LLTD for XP can be downloaded click here and the map will then work. This should identify if the problem is a network issue.