<a href="http://support.microsoft.com/kb/922120" target="_blank">Get it here. </a>
Ignore the fact that it mentions Vista. Hope it solves your problem.
<!--quoteo--><div class='quotetop'>QUOTE</div><div class='quotemain'><!--quotec-->This problem occurs if the Link-Layer Topology Discovery (LLTD) Responder component is not installed on the Windows XP-based computer.
Network Map in Windows Vista makes network diagrams of the connections between network devices that can be used to troubleshoot connectivity problems. Network Map uses a discovery protocol called Link-Layer Topology Discovery (LLTD) that queries other devices on the network to determine how the network is organized.
Windows Vista includes the software components that implement the LLTD protocol. For other devices on the network to be displayed on the map, they must enable a discovery protocol that can respond to the mapping requests from the Windows Vista-based computer.<!--QuoteEnd--></div><!--QuoteEEnd-->