Hi
I can't help much with Virtualbox, I did install W10 in VBox to start with but changed my mind and dual booted it instead.
It sound like your computer is trying to upgrade or overwrite your current operating system instead of installing it in VBox.
You might want to start all over from scratch remove and reinstall, VBox and try again, something must be messed up somewhere.
Here's some instructions that might help.
http://www.lehsys.com/2014/10/windows-10-installing-virtualbox/
Otherwise it's hard to guess at what's going on.
I actually prefer dual booting it since it lets you run a lot of your installed software without reinstalling everything and has access to everything just like your main OS
.
I've gone past that step now and am just using Windows 10 as my operating system, and running all my programs, games etc, from it.
It's pretty easy to create a 100Gb partition and just install it there.
I put all of the software that I did install for Windows 10 on a separate partition so that the install stays small and easy to back up.
The latest build seems pretty solid, but I do keep sytem images just in case.
Mike