A follow-up to our guide last week on running Windows 8 in a virtual machine: Microsoft has officially confirmed that while it's possible to run Windows 8 in Hyper-V, VMware Workstation and VirtualBox, it isn't an option right now in a bunch of older products, including several of its own releases.
In particular, you can't run Windows 8 under Virtual PC or Virtual Server, or in the 'XP Mode' of Windows 7. Microsoft's list doesn't include Parallels, which isn't surprising (it's hardly going to encourage migration to a Mac platform, after all). The post also explains in some detail why virtualisation compatibility isn't the first priority for the developer release.
Running Windows 8 Developer Preview in a virtual environment [Building Windows 8]