1. A method of protecting a client computer against malicious software, the method comprising: maintaining at the client computer a trusted storage set in the client computer, wherein the trusted storage set comprises files that have been deemed to be sufficiently unlikely to contain malware; detecting a file that is not part of the trusted storage set; creating a file identifier for the detected file using an application that resides on the client computer; sending the file identifier to a server for the server to determine if the server has a copy of the detected file stored thereon; sending a copy of the detected file to the server if the server indicates that a copy of the detected file is not already stored thereon; receiving from the server, that is external to the client computer, a scan report for the detected file, the scan report indicating whether malicious software was found in a copy of the detected file; and setting by the client computer access directives for the detected file based on the scan report.