One of the great benefits of virtualization software, such as VirtualBox, is the ability to clone an installation, use it, then remove it. Personally, I keep a backup of the original installation; up to date and loaded with common software. When I need a new machine for testing, I clone the original, reinitialize the MAC addresses, and get to work.
Unfortunately, a cloned machine will act as if it was never authenticated, since the clone process asssigns a new machine UUID. Thankfully, there is a simple workaround,
VirtualBox should be closed during the following process or it will fail.
First, locate the UUID of the original, located in the original .vbox file
Next, add the UUID attribute to the Hardware in the .vbox of the clone,
Once this is done, the machines will appear as identical and Windows will remain authenticated. In fact, this method even works if you choose to change the network MAC during the clone process (I usually do).
The proper way to do this is with the tools provided with VirtualBox, specifically 'VBoxManage modifyvm'
VBoxManage modifyvm [cloneuuid] --hardwareuuid [originaluuid]
However, the process outlined above has never failed and is much more efficient.