I encountered the same problem and solved it: Something was using the VT-x features, and according to a post on oracles forum (lost link, google a bit) vbox uses virtualisation in way not compatible with the former. You can confirm if this is the case by running msinfo32.exe
, scrolling to the bottom and look for a string that goes like "A hypervisor was detected..." as the very last line. If you see ~4 lines about hyper-v capabilities, your problem is different than mine, and the solution might not apply.
So now goes a potentially frustrating search for what is using virtualisation...
If hyper-v is installed and running (check in windows services) you can try and remove it; Control panel > programs > Turn Windows features on or off
People have also mentioned "other anti virus than BitDefender" and things running in XP compatibility mode as being the culprit.
I tried killing all processes and stopping all services - no luck (do this at your own risk, you might break something...)
Finally I was hinted to a new feature called "Virtualisation Based Security" in windows. Go to taskmanager and look under "details" tab for a process called Secure System
. If this runs, you've likely found the reason for your trouble; Despite removing hyper-v as described above, this feature runs some hypervisor way down deep, blocking virtualbox.
In regedit.exe set HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\DeviceGuard
to 0
and reboot. Did the trick for me. Beware that you're disabling a security feature, see https://technet.microsoft.com/en-us/itpro/windows/keep-secure/deploy-device-guard-enable-virtualization-based-security for details.