Just for fun, you could try disabling the Link Layer Topology Mapper and Scanner options in each NIC's properties. If this doesn't make any changes, you can re-enable it. I had to do this on our network if a Vista machine decided to show up, but my network copying issues ceased once I had a Server 2008 Domain Controller because it could respond to the clients with those services enabled. Before having that server, those clients would just keep flooding the network with packets, continually trying to map the network. This might be why Windows 7 has network discovery off by default. Just an idea.