If you ever try to start a Vista (or 2008) Virtual Machine (Virtual PC) that has been sysprep, you probably noticed that The VM additions no longer works.
Mike Kolitz found a solution to that problem. You should set the PersistAllDeviceInstalls flag to true in the generalize pass of your unattend.xml. For more details read Mike's Virtual Varia Blog.
If you you can read french you could also take a look at the document «Création d’une machine virtuelle générique (master) de Vista/Windows Server 2008 avec les outils de l’AIK» from Jean-Benoit PAUX.
Source : blogs.msdn.com/mikekol