A programmable computer processing apparatus is connected to a number of servers. Each server stores one or more separate components which, when combined, make up a processing application. Each component specifies any other components which it requires. Computer is arranged to identify a first component needed by an application requested by a user, to fetch the first component from the server storing the component, to fetch any additional components needed by the first component, and to continue fetching components until all of the components required for the application have been fetched. Computer is arranged to install the components in a Java virtual machine using a different classloader for each component and to perform control to permit inter-component interaction and interaction between the components and the resources of computer only in accordance with predefined rules. This prevents unauthorized access by a component to the functionality of another component or computer. Components may be downloaded from servers to take account of upgrades in the components.