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 head end node; and a plurality of capture nodes each of which is operatively connected to the head end node; wherein: the head end node includes computer hardware for performing central configuration control of the plurality of capture nodes; each capture node is operable to receive configuration instructions from the head end node to determine what simple event reports are to be generated by the capture node and in response to what conditions detected on the monitored computer network; the head end node 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 for causing the specified capture node to operate in accordance with the specified configuration instructions; and one or more of the capture nodes is configurable into a plurality of different configurations including a non-invasive mode of operation configuration and an invasive or minimally invasive mode of operation configuration.