1. An anti-virus file scanning system for computer files comprising: a) a computer database containing records of known executable programs which are deemed to be not malware and criteria by which a file being processed can be determined to be an instance of one of those programs; b) means for processing a file being transferred between computers to determine whether the file matches the criteria characterising a file as an unchanged instance of a program in the database; and c) means for signalling the file as known or not depending on the determination made by the processing means.