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 said server, 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; and if, as a result of the cross-referencing, a file that was previously classified as posing a threat is classified as not posing a threat sending an instruction to said client computing device or to the administrator responsible for said client computing device that said file is available for use by said client computing device.