1. A method for providing a computerized system which is protected from malicious programs coming from an external source, the method comprises the steps of: a) secretly, and in a manner unknown to authors of external programs, modifying on a temporal basis at a central security authority one or more essential elements of the operating system of the protected system in a manner which causes all running programs to fail, unless they are subjected to a compatible modification which enables them to run properly; and b) modifying also on a temporal basis at said a central security authority each program which is known to be benign in order to comply with said modification of one or more essential elements, thereby to enable proper execution of the program, and running the modified program at the computerized system