After moving to Windows 7, I wanted to move virtual machine to new Virtual PC 2007 SP1 installation.
Suddenly, Virtual PC displayed this message: “The virtual machine “<VPC Name>” can’t be started because it is being used by another program, or marked as read-only. To start this virtual machine, stop using the virtual machine in another program, or ensure that it is not marked as read-only.” .
The problem is well known.
The solution may be found here: Having problem opening VPC in Vista?
Here are few steps, I copied from source:
1. Right click on your Microsoft Virtual PC shortcut from the programs.
2. Select on “Properties”.
3. Select on the “Compatibility” tab.
4. Check on the “Privilege Level” to run the application as an Administrator.
5. Start the VPC and you’re good to go!!!