No longer seeing XP computers on network

So, I know this topic has been covered many many times, but this is slightly different.

I had all the computers on the network, a mix of XP SP3, vista and 7 pro of the 32 and b4bit flavors networked and working fine. All of a sudden, i can no longer see the xp computers from the win7, and vice versa. So i thought maybe there was an update that changed the node type (it's happened before on a win7 home comp in the network). All the nodes are hybrid, all still have file sharing and network discovery enabled and my LAN manager is still set to send lm&ntlm.

What's strange is that I can type in the computer name into the address bar and access it fine. But they won't show up on the network map. So, while for a techies like us, this is no problem, anyone who runs IT knows how the users like to have everything the way it was.... So I'm trying to figure out how to get all the computers back on the network map. Ideas?

so its a p2p and both win7 AND win xp sp3 suddenly one day are not visible to one another (win 7 map is completely blank or does it show at least the current computer (and I assume a router) connected to the Internet ?

did this just start, like for instance within a day

and no changes or additions at the same time (i.e. no firewall ports changed?)

Mike

the network maps are not completely blank. the xp computers can see each other (and themselves and the router), and the 7 computers can see each other (and themselves and the router) on the same side of the router. by that i mean, i have a wireless router feeding to several switches, some of which have an additional switch on the back-end. all the 7 computers feeding into one of the router ports can see each other, but they can't see the 7 computers that are being fed by a different router port. but if i click the see full network map, it can find the additional 7 computers, but isn't exactly sure where in the network they are.

as for any changes, none that i am aware of. i opened a port for vpn a few weeks ago, but this just started monday.