to install LLTD on to the XP system as, "This problem occurs if the Link-Layer Topology Discovery (LLTD) Responder component is not installed on the Windows XP-based computer".
Look at the two links below, (the 2nd link applies to Vista but it is the same for Win 7), and see if that helps;
Networking Windows 7 and XP
and Network Map in Windows Vista does not display computers that are running Windows XP.
A couple of other points;
1] Make sure networking is set up on both computers using Windows accounts that are exactly the same, ie, the same username and the same password.
2] Use the same Workgroup name
3] Ensure no firewall is blocking access.
Hope that helps.
Mark
I have a home network. On the network I have a desk top running windows XP Home Edition, and a laptop running Windows 7. I can see both computers on the network, and I can connect to the Windows 7 computer from the XP. I can not connect to the XP from the windows 7. I can ping both computers IP address and name. I am running Norton 360 on both computers. I am pretty sure there are shared folders set up correctly on both. Any ideas on what computer is causing the problem or what settings need to be corrected?

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic