MED-V 1.0 SP1: Windows Firewall blocks VPC 2007 network activity by default for standard users
By default, Windows firewall blocks Virtual PC 2007 SP1 network activity, and when Virtual PC 2007 SP1 initiates on the client computer, there is a firewall message that blocks its startup sequence and all network access. If you are logged on to the machine as an administrator, you have the option to enable these exceptions.
For newly deployed MED-V Clients where the user logging for the first time are not local Administrators, they will not have this option and they will not have virtual machine network access
The cause of this is an issue by design with Virtual PC 2007 in Windows Vista and Windows 7.
Right now, the current solution is to update the firewall exception by using Group Policy before MED-V is used by the end user. a Domain level group policy is probably the most recommended route to take here.