well, if you have it installed on your system using that cd-key you can look in the registry. it should be under something like
hkey_current_user->software->valve->(somewhere in here should be something labeled "key" and that will give you your value. it might not be in the valve folder, it might be in one of the sub folders, but it should be there.)
now, this is all if you have it installed. if not, i'm not sure how you can find it...