New Install - Forgot Key - In _old Folder From Previous Install?

my Windows 7 professional install seemed kind of lame after some years so I decided to install a new and clean one. I noted down the product-ID from my computers properties because I thought that was the key (right next to it it says change key, so I thought that must be the key).After the new install I realied that it is not equivalent to the key as the ID has less letters.Is there a chance to get the key from the "old" folder that is generated if you install a new windows over an old one?Or is there a chance to get to the key from the product ID? because I have no clue where I put the key..