Scanning for computer viruses or E-mail and data content filtering is performed using a distributed programming approach. A master computer 4 serves to divide the scanning operation into a plurality of tasks that are allocated to further computers 8, 10, 12, 14, 20. These further computers then separately perform the tasks and return the results to the master computer 4. The master computer 4 can check the update status of the further computers prior to them starting operation in order to check that they have the latest data defining the scanning to be performed.