For Microsoft Windows OS, default Storage is in %HOMEDRIVE%%HOMEPATH%\VirtualBox VMs
Which typically expands out to
C:\Users\[username]\VirtualBox VMs
The subfolders will be named with the Name: field contents entered in the Create Virtual Machine dialog where you're asked to enter Name and OS information.
If you've decided to do a little more orgainization, the Machine Name subfolders will be under the Machine Group subfolder. Full implementation may look like the following:
C:\Users\[username]\VirtualBox VMs\[vm group]\[vm name]
The VDI may exist elsewhere if you used the option to Use an existing virtual hard drive
It's easy to find where the VDI disks reside by using File=>Virtual Media Manager
