The only legitimate way is to purchase a copy of windows from the Microsoft store online, or via one of our retailers such as PB Tech (mentioned, who have a massive sale on RIGHT NOW- till THU 9pm), Dick Smith, JB Hifi, Harvey Norman, etc.
BizSpark is a special program for qualified Startups only, and the MSDN licenses are purely for Dev/Test - you are not allowed to 'run your day to day' stuff on MSDN licenses.
An OEM license can only be purchased and used to install onto a physical machine, and it is only ever valid for the machine you install it on. It cannot be transferred to another PC later if you change hardware.
There is an evaluation version of Windows 8.1 Enterprise only. If you install that and then try to downgrade to a retail licence later, you'll have to start over. I wouldn't recommend it if you plan on keeping the VM.
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