When I wish to start over with a fresh install of WinXP that I keep around (whenever Windows XP gets fouled up and I want to start over), I create a new profile and import a stock .vdi I keep around. When I attempt to add the stock .vdi to the profile, Virtualbox freaks out with a UUID error saying it “failed to open a session for the virtual machine” and “UUID does not match value”. Get around this by going to File->Virtual Media Manager…, delete the .vdi from the list, add it back in, and then go in to the Machine profile and re-specify the .vdi.