1. A computer system comprising at least one CPU, memory, non-volatile storage, an external interface subsystem for receiving electronic data from sources outside the system, a host operating system and at least one client application program, in which said external interface subsystem for receiving electronic data is at least one protective virtual machine (PVM); and in which at least one client application program and said host operating system are configured such that said PVM is the only path for data from sources outside the computer system to enter the computer system; said PVM contains at least one program for receiving electronic data; and said computer system comprises program means for terminating said PVM upon the occurrence of a triggering event.