1. In a computer system having an electronic memory and an operating system which assigns separate process address spaces for each application program which is executed, a method for modifying an operating feature of an existing target software application, without recompiling the source code for said target software application, said method comprising: executing an injection software application program simultaneously with said target software application program, said injection software application program having an emulation procedure; mapping a library function containing a surrogate procedure to a process address space assigned by said operating system to said target software application; retrieving from said electronic memory a first pointer to a target procedure for controlling a target application function; replacing said first pointer in electronic memory with a second pointer to said surrogate procedure, so that said surrogate procedure receives intercepted messages from said operating system that would otherwise be received by said target procedure; processing the intercepted messages from said operating system to determine if any of said intercepted messages requires processing by said surrogate procedure to modify said operating feature of said target application program.