Normally you always need VT-x for any Windows 8 OS and later. But I could be wrong here.
As Windows 8.1 is not a supported OS at this time, please see the dedicated topic to 8.1
, and this topic
VirtualBox VM 4.2.16 r86992 linux.amd64 (Jul 4 2013 15:15:20) release log
00:00:00.373383 Oracle VM VirtualBox Extension Pack (Version: 4.2.14 r86644; VRDE Module: VBoxVRDP)
Also to avoid possible future problems, make sure you upgrade your Extension Pack to the matching version of Virtualbox.