You can install any modern Windows or Linux version, desktop or server, using Hyper-V in Windows 8.1. But you'll need to bring your own license and software for the base OS. Here's how to get that OS cheap or even free.
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How well does the latest beta of Microsoft's virtualization solution work? In this gallery, Ed Bott looks at Hyper-V in action on Windows Server 2008.
This is so new that I can’t even find a press release about it yet, but Microsoft Virtual PC 2004 is now free. Anyone can download the latest release, Microsoft Virtual PC 2004 Service Pack 1, which runs on Windows XP Professional, Windows 2000 Professional SP4, and Windows XP Tablet PC Edition or later.