A repair system is disclosed comprising: a server having a repair manager; a client device having a repair agent; and a network, wherein the server and the client device are connected to the network. The repair agent, in response to a repair signal, is operable to restrict communication of the client device on the network to the repair manager, and the repair manager is operable to provide a repair application. The repair system is enabled to correct actual or potential problems including software viruses and trojans, and spam messages. Diagnosis and repairs can be carried out in response to a deleterious detection system or through user input to the device. The repair agent requires only limited system resources and therefore it particularly applicable to mobile devices where system resources are at a premium.