virtualbox.org

I have upgraded to Windows 8.1 today and installed VB version 4.2.18.

None of my previously existing machines will start (they are all 64 bit). A message box appears "VT-x / AMD-V hardware acceleration is not available on your system". I have VT-x turned on in BIOS. They all worked at windows 8 although I don't recall the VB version it was fairly recent.

Within the manager I have 1 processor set and can not increase it - it is greyed out - saying I need hardware virtualization support which I do have turned on.

Is there anything I can do?

The error from the log is (full log enclosed).

00:00:01.704490 HWACCM: No VT-x or AMD-V CPU extension found. Reason VERR_VMX_NO_VMX
00:00:01.704518 HWACCM: VMX MSR_IA32_FEATURE_CONTROL=0
00:00:01.704526 VMSetError: D:\tinderbox\win-4.2\src\VBox\VMM\VMMR3\HWACCM.cpp(737) int __cdecl hwaccmR3InitFinalizeR0(struct VM *); rc=VERR_VMX_NO_VMX
00:00:01.704528 VMSetError: VT-x is not available.

.
.

00:00:01.873749 ERROR [COM]: aRC=E_FAIL (0x80004005) aIID={db7ab4ca-2a3f-4183-9243-c1208da92392} aComponent={Console} aText={VT-x is not available. (VERR_VMX_NO_VMX)}, preserve=false
00:00:01.985967 Power up failed (vrc=VERR_VMX_NO_VMX, rc=E_FAIL (0X80004005))

EDIT: I removed Hyper-V from windows features and then it works now. Is it normal though that you can't run Hyper-v and VB at the same time?

Yes. If you enable Hyper-V, the hypervisor loads before Windows so VT-x is locked by Hyper-V at boot time. The only way to stop it, short of removing Hyper-V, is to use BCDEdit to change the hypervisorlaunchtype setting. Windows will then boot without the hypervisor.

If you wanted to run both VirtualBox and Hyper-V you would need two different boot settings, and reboot to run the other hypervisor.