When you install Virtualbox on Windows Vista or Windows 7 it will install an additional virtual network adapter on the host system. This “Virtualbox Host-Only Adapter” causes the Windows network detection to show an additional “unidentified” network. And while this “unidentified” network is present the windows firewall settings will stay on “Public”. This behavior is undesirable because you want to be able to switch your firewall settings based on your location (home/work/public).
The solution: Edit your registry and make that networks a NdisDeviceType. Open regedit and navigate to HKLM\SYSTEM\CurrentControlSet\Control\Class\{4D36E972-E325-11CE-BFC1-08002BE10318}
Browse through the subkeys (named 0000, 0001, etc) until you find the subkey containing the virtualbox network adapter, this is the one where the “DriverDesc” key has “VirtualBox Host-Only Ethernet Adapter” as value. Add a new DWORD value with a name of “*NdisDeviceType” and a value of “1” Disable en re-enable the virtuabox host-only network adapter.
Why that works? From http://msdn.microsoft.com/en-us/library/bb201634.aspx
*NdisDeviceTypeThe type of the device. The default value is zero, which indicates a standard networking device that connects to a network. Set *NdisDeviceType to NDIS_DEVICE_TYPE_ENDPOINT (1) if this device is an endpoint device and is not a true network interface that connects to a network. For example, you must specify NDIS_DEVICE_TYPE_ENDPOINT for devices such as smart phones that use a networking infrastructure to communicate to the local computer system but do not provide connectivity to an external network.