In your home directory you have a hidden directory .VirtualBox,
all your machines are contained in there. Take a look at that
directory and I’m sure you’ll understand how to manage that.
I’m old and lazy and have only one virtual machine so I always
keep a couple of copys of that directory on an external hard
drive, different instances. That way I can restore them on a
new installation, and also roll back to a previos instance should
something nasty happen to my XP-machine. Back up the XP-machine
you already have and feel free to experiment.
Select create New machine and then point it to the VDI file of the machine from your old files. Ideally you want to have already put the old VDI files in your current VDI location. Which as has been mentioned is by default it your hidden folders under your username.