Windows – Why is Windows 7 computer not visible in network

Try this on the Windows 7 machine:

1. Open Administrative Tools -> Local Security Policy.
2. Click on Local Policies -> User Rights Assignment on the left.
3. Double-click "Deny access to this computer from network".
4. Select Guest and click Remove.
5. click OK.
6. EDIT: Ensure in the Permissions for the network share, that Guest is included, since it's no longer part of the Everyone group.
7. Use regedit to go to HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Policies\System and create or modify the value of LocalAccountTokenFilterPolicy (32-bit dword) to 1, so the remote logon token will not be filtered (see here).

Check firewall settings or other software that may in any way interfere with network communication to/from that one system.

In order for a system to show up in network neighborhood it needs to be responding to the network broadcasts sent out by the other Windows systems. Basically, the systems send out pings to one another to see who else is on the network and if someone doesn't reply, the computers don't know they exist. If you explicitly try to access them however, it works and then for a while, your system will know they exist - but the next round of pings and it may forget agian.

It could also have to do with the versions of the systems. The Windows network protocol uses an elective system that weighs the OS as the qualifier as to who should be the network boss. Chances are your two Windows 7 systems should be fighting it out, but if some of your XP systems are trying to be the boss as well, it could be there's a constant re-negotiation for who the boss should be, so that one system constantly gets the losing end of it and doesn't show up.

Check your firewall settings though -I think that may be it.