Its a long shot that many people will have this issue but hey why not!
If you have one of the uber new AMD FX Processors (e.g. the 6 core 6100BE) and have tried to run a Windows Server 2008 R2 virtual machine on it you may have seen this error message.
An error occurred while attempting to start the selected virtual machine(s).
‘<VM name>’ could not initialize.
The virtual machine could not be started because the hypervisor is not running.
As such you can sort this issue out by running the command bcdedit /set xsavedisable 1 at a elevated command prompt.
Just make sure you restart your PC after and boom you will be able to run your virtual machines again!