@ Zhangm: It doesn't come up in start ->search. I checked Control Panel -> add Features and anything hyper-v isn't listed.
Its right in program and features - under turn windows features on and off right on the left menu
You will then need to make sure these are installed
Then the command is there in powershell, if you want to use virtualbox's vboxmanage - then you need to install virtualbox, not some 3rd party tool. vboxmanage is just a cmd line tool for use in virtualbox, here is link to manual on it
edit: Another option is why do you need to convert it? Why not just mount it on system that understand vhdx and pull your files off? Seems more work to do a convert on the whole thing if you just need a few files off it.