1. A method for detecting intrusions in the operation of a computer system comprising: (a) gathering features from records of normal processes that access the file system of the computer; (b) generating a probabilistic model of normal computer system usage based on occurrences of the features and determining the likelihood of observing an event that was not observed during the gathering of features from the records of normal processes; and (c) analyzing features from a record of a process that accesses the file system to detect deviations from normal computer system usage to determine whether the access to the file system is an anomaly.