A couple of things to check. First make sure the XP machine has the LLTD responder. Here is the site to download.
Network Map in Windows Vista does not display computers that are running Windows XP
Next; Firewall and firewall settings. Norton and CA are known to have issues with networking computers. I was using CA firewall and anti-virus and it was giving me fits so I switched. Now no problems. Make sure your firewall is allowing access to network. Make sure you have set files you want to share set to sharing.