When I try to register my CD key, Steam says it's already in use.

This error message means that somebody has already registered your CD key using a different Steam account.

• I've used this CD key in Steam before.
• I've had this CD key for a while, but I haven't used it in Steam before.
• I got this CD key from a game that I bought used.
• I got this CD key from a friend or website.
• I got this CD key from a game that I just bought new.