Hi there smarr.
Im afraid that with this sort of thing, its going to be a bit of guesswork. However, here are a few things that could be causing it:
1. Windows is just being weird
2. your Firewall. Any Firewall (with the exception of the Windows Firewall, which (NORMALLY sorts itself out properly) can cause all sorts of issues with networking,
3. (I think this one is the most likely) Dont suppose you have your Networks on the Win 7 machines set to "Home" do you? Change it to Work
Occasionally Windows 7 confuses itself and thinks it should be in a Homegroup even when it isn't configured when set to "Home"
4. Update your Networking drivers
Thats all I can think of for now