Eugene's observations

The word on the street is that Windows 9 will, as OS X and Linux have for years, support virtual desktops ? the ability to group many programs? windows and move the whole lot on/off the display together.  The functionality is available now free by running Sysinternals Desktops.

But the thrust of this item is virtualisation ? the ability to run one operating system inside another.  This is often useful for running old programs you don?t have a licence to run on modern versions of Windows, or testing out a possibly virus laden program/website, or running a Windows program side-by-side with an OS X one, or even as a form of image backup.

There are five heavyweight players in this space and each has their pros & cons:

• Microsoft?s Hyper-V
  Available only with the Professional editions of Windows 8 but included without further charge.  The VHD format guest disks can be mounted for read/write in the host OS to enable easy file transfer and virus scanning.
• VMware Player & Converter
  Free, hence limited support.  The Converter is a physical to virtual tool that easily bundles up a real system for transportation elsewhere and execution on a Player.  Converter can even bundle up Acronis .TIB images (old versions only).   
• VMware Fusion & Parallels Desktop
  Want to run Windows on a Mac ?  Apple?s Boot Camp allows you to boot into one or the other but virtualisation allows you to run them both together.  Fusion runs Windows well but does not allow OS X guests.  This is true of competitor product Parallels Desktop also.  Parallels is a slicker experience with good support but unlike Fusion cannot be run as a VM within Player.
• Oracle?s VirtualBox
  This is free and runs on Windows, OS X & Linux but is sometimes hard to work with and good support is hard to come by.  Other virtual machine containers supporting the Open Virtualization Format can be imported but going the other way is difficult. Building guest OS?s is easy but switching hosts is hard work.  There is a portable version enabling you to take the virural machine with you on a USB drive and use it on any other computer.  Probably the best solution if creating many snapshots is your goal, but to maintain a sandboxed environment don't enable file sharing, use bridging to connect to a real network, or use regular accounts (that synchronize stuff).
• Microsoft Virtual PC
  This is part of the 'XP Mode' that is incorporated into pro editions of Windows 7 but can also be coaxed to work under Windows 8 now renamed to Hyper-V

Remember that both host and guest operating systems will need a valid license to work properly; moving a guest to a different physical machine is possible but the hypervisors change the MAC address of the network card so reactivation is usually necessary.   Also, you cannot have too much RAM for smooth running virtualisation.