1. An anti-malware file scanning system for computer files being transferred between computers, the system being implemented on a computer apparatus and 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, the criteria including at least one characteristic signature associated with each said instance; b) means for processing a file being transferred between computers, the means b) comprising: a file recogniser operative to determine whether the file being processed is an instance of a known program by checking the contents of the file being processed for the presence of said at least one characteristic signature associated with the said instances; a difference checker operative, in the case that the file recogniser determines the file being processed to be an instance of a known program, to check whether the file is an unchanged version of that known program; c) means for signalling the file, depending on the determination made by the processing means, as being: likely to be not malware if it is an unchanged version of a known file; likely to be malware if it is a changed version of a known file; or of unknown status if it is not determined as being an instance of a known file; wherein the processor assigns a score to a file identified as likely to be malware, and storing the determination that the file is likely to be not malware, is likely to be malware or is of unknown status.