1. A method for the retroactive detection and removal of undesirable software, comprising: periodically receiving, at a server in communication with a network, information relating to files on a client computing device, including information relating to whether one or more files have previously been classified as posing a threat or not; storing on said server said information relating to files on said client computing device, including said information relating to whether one or more files have previously been classified as posing a threat or not; periodically receiving at a server in communication with a network information useful in classifying files as posing a threat or not; cross-referencing said information relating to files on said client computing device with said information useful in classifying files as posing a threat or not to determine whether a previous classification of a file is inconsistent with a classification based on current information; if an inconsistent classification is found; communicating information concerning an updated classification to said client computing device or to an administrator responsible for said client computing device.