gguenette, since you're not a real MS tester you're trying to use a image backup of the new CTP. well microsoft released the image installer (modified win pe) with the new release. it must be put onto disc and run from there. it loads it's own non-MSDOS installer to configure Vista.
if you have a copy of the original installer, then it has to be put on disc. if you have a rip of the original, then you're screwed.