It is highly recommended that a sub-floor be installed when laying new hardwood flooring. If there is an existing sub-floor that squeaks, simply screw long drywall screws where it is squeaking.
The suggested minimum requirements for a sub-floor under hardwood flooring is 3/4 inch plywood that is solid and free of water stains and squeaks. If the existing sub-floor is thinner than 3/4 inches, simply lay another sheet of plywood on top of the existing sub-floor and secure it with drywall screws. It is also suggested that the sub-floor be swept or vacuumed thoroughly before laying the new hardwood flooring down.