I got this error after installing Visual Studio 2015 CTP 6 with Windows Phone 8.1 emulator. Sometimes you can get this error if you turned off VTx or AMDv in bios. So, first things to do is: make sure that Virtualization is enabled in bios.
Next - turn off Hyper-V. After installing Visual Studio with emulator it automatically turn Hyper-V on. And, apparently, Virtual Box cannot work together with Hyper-V.
- Run cmd.exe as administrator
- Execute command
dism /online /disable-feature /featurename:Microsoft-Hyper-V
- Restart computer