1. A method for authenticating a user on a computer system comprising: installing on the computer system a sensor for detecting a plurality of different types of user interactions with the computer system including process creation, process destruction, file touches and registry actions, for a plurality of users, measuring with the sensor how many of the plurality of different types of user interactions occur during a time period; evaluating the usability of the different types of user interactions that are measured to discriminate among the different users by identifying those user interactions having low variance over time with the same user and high variance over time between different users; using at least some of the different types of user interactions that have greater discriminatory significance to train a model of user interactions for each of the plurality of users; monitoring the interaction of a plurality of users of the computer system during a second time period subsequent to the training of the models to measure for each user how many of the plurality of user interactions with the computer system occur; and comparing with the models a pattern of behavior generated by a user of the computer system to determine if the user who generated the pattern of behavior is the user represented by one of the models.