1. A complex event processing system comprising a complex event processing engine and an event harvesting system, wherein the event harvesting system is operable to monitor a computer network, generate simple event reports in response to the result of monitoring the network and pass these to the complex event processing engine for processing, wherein the event harvesting system comprises: a central configuration control module; and a plurality of capture node modules each of which is operatively connected to the central configuration control module; wherein each capture node module is operable to receive configuration instructions from the central configuration control module to determine what simple event reports are to be generated by the module and in response to what conditions detected on the monitored computer network, and wherein the central configuration control module includes an interface for receiving configuration instructions from a user of the system and for processing these configuration instructions and sending them to a specified capture node module for causing the module to operate in accordance with the specified configuration instructions.