VirtualBox is a very easy to use hypervisor that was initially created by the legendary SUN Microsystems who was later bought out by Oracle, thus giving VirtualBox the name Oracle VirtualBox. It can run any OS and has integrated acceleration drivers to allow for faster graphics and resolution switching.

If you have somehow enabled Hyper-V or some other backend, it may turn out that you cannot use VirtualBox any longer. To turn off Hyper-V, you must turn it off by using the Windows Features dialog.

After restart, open run the following command as admin:

bcdedit /set hypervisorlaunchtype off

Restart once again and VirtualBox should be functional again.