Either the purpose of the agreement or the means by which it is accomplished must be illegal to support criminal prosecution on conspiracy charges. If the purpose is unlawful, the offense is committed even if the means used to achieve the purpose are lawful.
One illustration is where a noncustodial parent conspires with another person to kidnap the parent's child, and the child is abducted during a court-approved visit. Conspiracy also occurs if the purpose of the agreement is lawful but the means used to achieve it are illegal. For example, if a custodial parent chooses to retrieve a child who has been kidnapped by the noncustodial parent, an agreement to use unlawful force constitutes conspiracy.
http://www.answers.com/topic/conspiracy