A distributed computer system is disclosed in which computers co-operate with one another by sending messages over a network such as the Internet in order to perform a distributed application. In order to improve the security of such system, each web service involved in the distributed application runs in a separate virtual machine. Furthermore, the virtual machines on a web server dedicated to respective web service instances utilise the same policy enforcement point—running in another virtual machine on the web-server—in order to handle messages for or from the web server. To increase security still further, each virtual machine provides virtual cryptoprocessor functionality which is used in the processing of messages sent in the performance of the distributed application.