Please Help!

I purchased a copy of Just Cause 2 today from a charity shop and when I returned home, I inserted the disc in my PC but the product key that I entered has already been used through Steam (more than likely by the original owner). Therefore I am unable to play the game at all.

I have raised a query with Steam directly but I doubt they will be quick coming back to me. Therefore I was wondering if anyone has a spare product key that I can have?

Thanks in advance