>>>One thing that's handy is to copy the VHD file (after shutting down the VM) once you've got a base configuration set up. That way, you could easily roll back the VM to a particular state by copying back the backup image. Especially useful in testing install to make sure it works correctly in a "clean" setup.
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>>The snapshotting capabilities of more advanced VMMs like VirtualBox make this sort of testing very easy. With very little effort you can retain snapshots of various intermediate states while testing involved processes or installations.
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>In addition, you can have 'independent' disks in two modes
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>( I haven't used non persistent disks)
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>(1) Persistent - Disks in persistent mode behave like conventional disks on a physical computer. All data written to a disk in persistent mode is written permanently to the disk.
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>(2) Nonpersistent – Changes to disks in nonpersistent mode are discarded when you power off or reset the virtual machine. Nonpersistent mode enables you to restart the virtual machine with a virtual disk in the same state every time. Changes to the disk are written to and read from a redo log file that is deleted when you power off or reset the virtual machine.

Interesting. It looks like persistent/non persistent is VMWare parlance; the VirtualBox equivalent to non persistent seems to be "immutable". http://www.virtualbox.org/manual/ch05.html

>>Although I haven't tried it yet, I believe with VirtualBox you can even take a snapshot of a running VM.