i can't find my product key

Solution
If software supplier legit and you downloaded the ISO, then they should supply the install key. If you physically have a DVD, then it should be printed on documentation. None of these, its pirated and you should inform Microsoft.
I think Belarc ought to show you what your key is. If that doesn't work then either call up the company or buy a new copy.

Are we talking about windows here? Also are you trying to put windows that was installed for you by the manufacturer onto a new system? If so that won't work.