After either installing a fresh Vista RC1 image or upgrading from the Beta, when launching Virtual PC, you might get this error message:
Access to the external network and host will be unavailable to all virtual machines using virtual networking. Virtual machines using virtual networking will still be able to access other virtual machines using virtual networking.
To fix this problem, re-enable the Virtual Machine Network Services driver on one or more ethernet adapters or reinstall Virtual PC.
To fix this, go to the Network and Sharing Center > Manage Network Connections and right click the active connection. Go to Properties> and select Install... Select Service and then Add. Select Virtual Machine Network Services and then OK.
This will enable the Virtual PC Network Capability to bind to your physical adapter.