virtualbox.org

I was using virtual box on my win8.1 64 bit syste, without any issues. But since I upgraded it to 4.3 version, every time I start a VM, it fails to start with the mesage ,VT-x is not available. (VERR_VMX_NO_VMX).

Please in future post log files as zipped attachments, not pasted text.

As to your problem, it really is quite simple: you have VT-x enabled in the settings of a VM, even though VT-x is not available on your host. In VirtualBox versions prior to 4.3.0 this configuration error was silently ignored, but that is no longer the case.

You can fix the problem by unchecking VT-x in the VM settings. If the GUI won't let you then you can do it via the command line. Make sure the VM is shut down and then type :- Without access to VT-x/AMD-v you will not be able to create 64bit guests, or Windows guests from Win8 and later. So, you might also want to read the FAQ: I have a 64bit host, but can't install 64bit guests.