I was trying to install Windows SBS 2008 (64-bit) for testing purposes, but I kept receiving this error "VT-x is not available. (VERR_VMX_NO_VMX)" from Virtual Box everytime I tried to Start the system.
I enabled Hardware VT from the BIOS (under Security menu) and restarted the computer - still the same error.
I disabled Execute Bit from the BIOS - still the same error.
I untick Enable VT-x/AMD-V in VirtualBox Settings->System->Acceleration - it gave a different error.
I even downloaded the SecurAble to double-check if my virtualization & DEP are enabled, same with the 64-bit capacity - and yes, I have virtualization & DEP enabled on my BIOS & it is 64-bit.
I also tried editing the XML file %systemdrive%\Users\%username%\.VirtualBox\Machines\<OS name>\<osname>.xml and type "false" for enabled of <HardwareVirtEx enabled="true" exclusive="false"/>
Shutting down the computer and leaving it off for 5 minutes fixed the issue. I have now Windows SBS 2008 (64-bit) working as guest OS of my Vista (64-bit) parent OS.