We have a large off-net (no connection to the internet or corp desktop net) Ethernet network. It has had mostly Win2K, WinXP nodes running proprietary apps. We have started introducing some Win7 nodes for various initiatives, and have had some of the same problems with the networking, i.e. the Win2k nodes can't see the Win 7 nodes, vice versa, and Win7 can't see Win 7.
The problem seems to be in the Win7 networking. Win7 does not seem to like workgroups. I think MSFT is trying to do away with this concept. If you choose 'Home' or 'Business' when you install the Win7, you have a devil of a time seeing anything in a workgroup.
I have been changing the Win7 nodes to 'public' networking, and turning off the 'advanced' security options. Probably not a good choice if you are connected to the Internet.