I think you should be OK, DJ, providing you are installing like for like (i.e. Pro for Pro), and it's on the same machine as the original licence. You will know that you can't use a 'Home' licence to install 'Pro'. I have used this method to install a clean 'Home' version before, when only the recovery CD was available, so I don't see why it shouldn't work with 'Pro'.
I don't think the licence key is where you said, but there is a way to find this, which I will try to find for you, unless someone beats me to it. Does the case not have a sticker on it with the licence key?