Microsoft changed the network interface between Windows XP and Windows Vista, and again between Windows Vista and Windows 7, and introduced some breaking changes in both transitions. The easiest way to get all machine types to talk is by making some changes on the Windows 7 computers.
There is some (lame) help info available from various sources:
Link 1[
^]
Link 2[
^]
... and of course Google can help with more details.