virtualbox.org

Windows 7 machine.
I have a VHD created from a system image.
Do I have to install the OS (Windows 7) into the VM to use the VHD?

If it is an image of a running system the OS will be included in the image. That does not necessarily mean the you can simply create a virtual machine using the existing .vhd and it will run. It might if the "hardware" in the vm is a close match to the harware of the system on which Windows 7 was installed, but it might give you a blank screen of a BSOD.

The biggest problem is selecting the proper HD.
The easiest way I found is to check the original windows device manager for the exact HD type and controller type.
When you create the VM (to represent the VHD), you need to select that same type of HD and an appropriate controller