From within Virtual PC
1. Select your virtual machine
2. Go to the Settings (either right-click the VM and select "Settings" or from within a running instance go to the "Edit" menu and select "Settings)
3. Click the "Networking" setting
4. For your network adapter, select "Microsoft Loopback Adapter"
Now, you should be able to see your other running Virtual PC instances that also share the same loopback adapter!
Note: If you run into a problem where you can’t select the Microsoft Loopback Adapter", you’ll need to verify that
1) The "Virtual Machine Networking Services" are installed for the loopback adapter, and
2) If they are, and you still can’t select the adapter, uninstall the service and then reinstall the service (by pointing to the Virtual PC installation directory under Utility/VMNetSrv).