I've had Virtual Box running for ages. I use it for experimenting... I also use it to host VPN'd OS'. The whole concept of having applications and OS' fixed into a container is fantastic.
I also dabble with Smart-phone development; usually using a Macintosh or Android Studio on Windows. I recently became aware that Visual Studio Community 2015 comes with the opportunity to code Android/iOS apps via Xamarin.
I've attempted Xamarin before, but never built anything productive. This was years ago and have since forgotten about it. Seeing this option in VS meant that MS were somehow backing it and I therefore took the plunge.
Installing it was fine and building a sample 'WebView' application harmless. It even ran successfully in the emulator!
Later in the day, I attempted to boot up my trusty VM to acquire a few things. I was greeted with the error that VT-x was not available. I attempted to re-configure the machine, but the Acceleration tab was disabled as Virtual Box could not actually find any acceleration to use.
It turns out that installing the Android Emulator for VS also installed Hyper-V Virtualisation. On any windows machine, only one Virtualisation manager can be installed and VirtualBox is not compatible with Hyper-V! There's a blog post here of someone asking the same questions.
I have found one blog post by Scott Hanselman that tells you how to 'multi-boot' into Windows where Hyper-V is disabled, but then you cannot run your Android Emulator. This post was actually from January in 2014! I'm surprised I've only just hit this issue.
It turns out that VirtualBox simply cannot work with Hyper-V. I'm not sure if the Android Emulator can work without it.
I attempted a switch to VMWare only to find that it is also not compatible with Hyper-V! At least it presents you with an appropriate message.
The solution to all of this is to use the Hyper-V Manager which is installed as part of Hyper-V and create virtual machines from there. I haven't done this yet, but am not expecting too many issues.