Abstract: A client computer maintains storage of a trusted set of files. For files that the client computer detects that are not part of the trusted set, a server scans copies of files to protect the client computer against malicious software. After scanning, the server sends to the client computer a scan report indicating in which, if any, of the files malicious software was found. The client computer then sets access directives for the files based on the scan report. The system may be configured so that, when the client computer detects a file that is not part of the trusted set, the client computer creates an identifier for the file and sends the identifier to the server. The server can use the file identifier to determine if it has a copy of the file, and only if it does not will it request a copy of the file.