A method and an apparatus for distinguishing humans from computers and for controlling access to network services. One intended application of the method is a CAPTCHA technique, deployed using a shared Trusted Computing technology over a trusted network of a user terminal, a network server, and a Trusted Party, any of which may be at a Decision Point. The method distinguishes a human user making a legitimate request for network access from a programmed computer making undesired requests, by detecting unusually high network access request frequencies made by an identifiable user and/or a trusted module from the user terminal. The CAPTCHA function is further used to improve the method for controlling access to network services. The information transmitted between the members of the trusted network may be encrypted.