Do you have "Virtual PC Network Filter Driver" ticked in the Properties for the Host's Network Adapter?
In the Settings for the Virtual Machines, are the settings under Networking correct where they indicate which Adapter (the Network Card etc, on the Host Machine) that should be used?
Looking at IE under Tools/ Internet Options/ Connections/ LAN Settings/ Proxy Server, on all machines, is there any setting showing in there?
Is the firewall on all machines the stock standard Windows Firewall or are they third party products?
Have you tried from a Command Prompt running the Netsh Winsock Reset command on both VM's?
Can all machines Ping each other?
Are there any clues from the Event Viewers on the VM's?