1. A computer security system for use in a network environment comprising at least a first group of user computers arranged to communicate over a network, the system comprising a warning message exchange system operable to allow the communication from the user computers of warning messages relating to suspect data identified as a possible security threat; a message counting system operable to maintain a count for every particular piece or set of suspect data based on the number of warning messages communicated relating thereto; and network security means operable to act in respect of any particular piece or set of suspect data for which the count maintained therefor is substantially equal to or greater than at least one threshold value.