A method and corresponding equipment by which software (11c) on a first device (11) is virally or otherwise distributed to a second device (12), in which the first device (11) and second device (12) and possibly also a server (16) of the software (11c) exchange information sufficient for the first device (11) to determine whether it includes or stores a version of the software (22c) compatible with the second device (12), and if so, then provides a compatible version, but otherwise exchanges information with the second device sufficient for the second device to obtain a copy from the server (16).