Being used VIRTUAL Machine, VIRTUAL BOX, VMware ~ have the following observations:
VIRTUAL PC: WINDOWS7's "free features". It is easy to combine other WINDOWS systems. It only supports single-core in the VM ~ VHD file can be directly mounted to boot as if a normal PC. In my opinion, the Windows 7 Virtual Machine feature is useless if you get either one of other two programs.
VIRTUAL BOX: SUN's product, forever OPEN SOURCE, forever free, and supports multi-core on any CPU number you want. Compilable with VHD and VMDK files~ It's a user's friendly program.
VMware: In my opinion, it is the best overall. The most interest is the part is that on a X86 system, you can VM 64X system, support multi-core on 2, 4, 8,16.