For example, you a create a base virtual machine with Windows XP SP3 that has all the service packs and updated hot-fixes. You now are tasked with researching different virus protection systems. A simple way to do this would be to create 3 child virtual machines and create a differencing disk that holds different virus protection systems.

The following diagram shows the configuration we will create.

Now that you know what you are tasked with, your fist step would be to make a copy of the existing Windows XP SP3 base configuration. After a copy is made, set the Parent VHD to read-only so you cannot accidently make changes to it.

Note: If you make changes to the Parent VHD, it will break the children per the diagram above.

Our next step is to create  3 new virtual machines, following the instructions from my previous post. The only thing that changes now is that you create a Differencing disk, as shown in below, and choose the Read-Only Parent VHD.

Now that you have all three vm’s created (see below). You can start each of them up and install the appropriate virus protection for testing.

The screenshot below shows all three Windows XP machines with Differencing disks configured with virus protection.

As you can see, working with Differencing disks in Windows Virtual PC can be very powerful. After creating my base (Parent) image, I was able to create the 3 virtual machines with Differencing disks in under 5 minutes.