Working with the standard virtual machines released by Microsoft you might experience like I have shutdowns of the machine (unexpected shutdowns I might say). Looking in the event viewer you will however notice this: "The license period for this installation of Windows has expired. The operating system will shutdown every hour.".
So, this explains it. Looking around for a solution I found you can rearm your windows on these virtual machines using slmgr (tool included with Windows):
slmgr /dlv (this will give you the info if you have rearms left, and if you have just do a)
As a side not, rearming means the (Windows) evaluation period is extended so you can properly understand the product (in our case of course it's the new AX).