Automatically Deleting a VDI DIF file

[CodeMastadon]

I am new to VBox, and have a couple questions...

My host OS is Win 7 and I have configured a Win XP SP3 image for running legacy applications. I have created an immutable image and have several VM's defined using the same image. My issues:

1. When I power off a VM, the difference file remains on the system until I restart the VM. This can waste several GB of space. Is there a way to cause the image to revert to initial conditions on shutdown instead of power-up

2. I would like to wipe the differencing file before it is deleted to ensure that confidential information does not reside on my HDD after shutdown. I am unable to find an option to erase the differencing file before it is reinitialized at the next session. Does such an option exist? Is there a tool?

3. Does V-Box offer a way to encrypt VDI images? This might be a good way to quickly clear an image.