The interaction of a plurality of users with a computer system is monitored and measurements are made of different features of this interaction such as process creation, registry key changes, and file system actions. These measurements are then analyzed to identify those features that are more discriminatory. The set of features is then used to develop for each user a model of his/her interaction with the computer system that can then be used to authenticate that user when interacting with the computer system at a later time. Advantageously, these steps are performed automatically and may be performed periodically or even continuously to verify that each user of the computer system is indeed the individual he/she purports to be. Illustratively, the feature extraction is performed using Fisher's criteria; and the user model is developed using a Gaussian mixture model. A method for updating the user model is also disclosed.