[Unattended withoud key]

Does anybody know if its possible that you can run an unattended install with the autounattend.xml without defining a product key??

So that at the first boot Vista asks for an product key?

Maybe helpfull what my final goal is

Sometime i need to install a pc with Vista, each pc has different hardware so i was thinking that an unattended install was the best option.

I also have thought of using sysprep but understand it not entirely

Maybe someone can also tell me when to use an unattended file and when to use sysprep