fenshome:
Hello and welcome to the Windows 7 Forums.
A couple things to try;
If I understand correctly your problem is that you can no longer see the XP machine from within the Win7 machine's network utility in Windows Explorer. This information is dependent upon the Win7 machine having up to date and accurate netbios information regarding the hostname and IP address of the XP machine.
1. On the Windows 7 machine open a command prompt with elevated privledges.
Start Orb, all programs, accessories, right click command prompt and choose run as administrator.
type nbtstat -R (case sensitive capital R) this will purge and reload the netbios cache table on the Win 7 machine, then type
ping ComputerNameOfXpMachine this has to resolve successfully in order for the netbios information to propagate properly
then type
nbtstat -c this will display the names that are present in the machines netbios cache table, you should see the XP machines name and IP address
Check and see if the XP machine is now visible to the Win7 machine.
2. Check and make sure that both machines have netbios over TCP/IP enabled in the properties of IPv4, advanced button, WINS tab, for all adapters in use.
3. Make sure that both machines have the Server service and Workstation service started and set to automatic.
4. On the Win7 machine check and make sure that the SSDP service is started.
5. Make sure on the XP machine that you haven't added or adjusted the settings for any third party firewall or internet security suite, that might be inhibiting the propagation of netbios information. Usually this is where I recommend uninstalling any such software until you've been able to exclude such software as the potential cause of the issue. Attempts to disable or turn off this software is not often a solution. You can always reinstall it later after the problem is resolved.