I've got a network of a few XP machines and 1 Windows 7 PC.
The Windows 7 unit doesn't see any of the XP machines. HomeGroup is disabled, workgroup is correct, and all IP's etc are proper. I can ping the other computers, but cannot see any of them in the Network area and nor can I connect to any printers that are on XP machines.
I do have some shared folders/files on my network, but I haven't even gotten there yet... the Win 7 machine doesn't see any of the computers on the network -- yet it can ping them all and has full connectivity.
No, I haven't done that for each folder.. but I shouldn't have to, just to see the networked computers. The printers that I am trying to access on the Win 7 machine are all shared properly, but the Win 7 machine doesn't see the other computers -- let alone allow file/printer sharing.
I know this is an older post and you may have already found a solution, but in order for W7 to be able to see the XP machines over the network, you should ensure that identical login/password profiles exist on all of the machines involved.
If there is a user profile on your XP rig called "User" with a password "password", and a user profile on your Vista/W7 rig called "User" with a password "password", then the two machines will be able to talk with each other. They will not be able to do so in other scenarios. This is a security feature, to prevent the looser security in XP compromising data security on your Vista/W7 machines.