I got the same problem when I tried to install Ubuntu 15.04 virtual machine using VirtualBox (ver. 4.3.28). I look around the internet to find out the solution for it, and I did find a very simple way to fix it.
1. Open your ‘problem’ vm settings windows.
2. Navigate to System section, stick on “Enable PAE/NX” box in Processor tab.
3. Run your vm again, and see ‘the magic’😀