I have used VirtualBox as my desktop vm tool on and off for a couple of years. It is easy to get going, especially compared to the free VMWare server option and has worked pretty well. I recently started having disk space problems though and wanted to move everything to a new HD. Seems to be a common problem. The default is to look in your home directory, like c:\Users\me\.VirtualBox and then use relative paths from there for Machines and especially for Sandboxes under a machine folder. You can set the machines folder for future vms, but not for existing. Seems others were having similar problems. Here is what I found.
After shutting down all my vms, I moved the whole structure to my new drive. Then I found an environment variable VBOX_USER_HOME that I set to the new root folder, e:\vms in my case. I set that and viola, worked just liked I hoped.