I bought Overture late last year from Amazon UK, but have only just got round to installing it. Firstly it would not install OpenAL and I had to download it. Then it asks for aCD key but I cannot see one anywhere on the box or CD.
Different version of the game has different publishers and different publisher have released different version of the game. So I'm afraid we don't really have any clue as to where/how/if to use a key.
Did you download any updates for Overture? If so there are some update that are specific for certain versions of the game and using those updates on the wrong version could result in a key free version to ask for a key after updated.
This update
http://support.frictionalgames.com/entry/77/ should probably work regardless of what version of Overture you have.