Win XP machines not visible to Win 7 machines within same network


I have a workgroup set up with about 10 computers: 6 running windows 7, and 4 running windows XP. This morning out of nowhere, the XP computers can't access the network (network is set up and hosted on Win 7 machine), and the Win 7 computers can't see the XP computers in the network.

4 things are happening that make this particularly weird:

1. from the Win 7 machines, I can ping the XP machines successfully, and from the XP machines I can ping the 7 machines successfully. They are both active on the network.

2. From the host computer (Win 7), I can see and access all of the XP computers, though the XP computers give me an error message when I try to even go so far as to find the Win 7 computers on the network.

3. From all the XP computers, I can see the other XP computers on the network, and from the Win 7 computers, I can see the other Win 7 computers.

and finally,

4. The Win 7 computer is hosting a database back end from which the front end on each of the computers (XP and 7) pulls the data. Both XP and 7 computers have no problem still pulling the data from the back end in the Shared Documents folder on the host computer, even though the XP computers can't see the network.

Any suggestions?
Sometimes I have problems that appear similar to your ones:
I can access PC A from PC B but not PC B from PC A.
PC A also can't access the shared workgroup, but PC B can and can see all PCs inside (including those that can't access the workgroup^^).
PC B is always running like a server, PC A is being restarted every vew days.
It seems like the RPC Host Process service is broken and cannot be restarted on PC B.
To fix it, I usually restart at least PC B, but usually turn off all PCs, restart the router and turn all PCs on again. So I'd suggest the same to you.

I also haven't found a real solution to this and simply accepted that I can't use windows file access for more than a few days after a reboot.
That is an odd issue I've not seen before. Long shot, but change the network switch to another one.

And this is another reason to have an Acronis (or other imager) backup for each PC. If I ever experience your issue, I'd backup data as needed and then restore the appropriate image on each PC to revert to time when everything was working properly (make sure to not boot any machine until all have been reimaged). Someone really smart might know the proper solution, but I'm dumb and just brute force fix stuff with Acronis (usually takes less time too).