The virtual disc images we get from Microsoft usually have the same UUID as earlier disc images from Microsoft.
This is a problem if you wan't to run several images with VirtualBox because VirtualBox assumes, well that the different discs have different UUID's.
Here's how to change the UUID of a new disc image.
Open a command prompt and go to the VirtualBox program folder. This is most likely C:\Program Files\Oracle\VirtualBox
Run the vboxmanage internalcommands sethduuid [file] command. For example:
vboxmanage internalcommands sethduuid "C:\DEMO\AX2012-A.VHD"