It sets HKEY_LOCAL_MACHINE\SYSTEM\ControlSet001\Control\Lsa\limitblankpassworduse = 0. The default in XP is 1. Virtualbox's guest controls let you run commands, copy files, etc in the guest VM from the command line of the host VM, but it cannot communicate with the guest if the user has a blank password by default. ievms needs the guest control to work so it can launch the IE7/IE8 installers, so a tiny custom linux iso is booted that mounts the VM hdd and makes the single registry change. Once that's done, ievms can take over from the host command line.