I don't think there's a way to do this with the new VirtualPC without having VirtualPC loaded and running and doing it from within whatever PC you're running.
In VirtualPC 3.0, I remember being able to double-click on my VirtualPC drive image and have it mount on my desktop, enabling me to find files in my VirtualPC's hard drive without having to boot VirtualPC. Since then, that functionality has been lost... (sniff)