1. A security system for efficiently monitoring and securing a computer including a memory and processor, to defend against malicious intrusions, the system comprising: an in-band software monitor disposed within a kernel in communication with an operating system (OS) of the computer, the monitor configured to intercept system calls made from an MSR (Model Specific Register) of the processor, to execute monitoring operations, and to subsequently return execution to the OS; an out-of-band hypervisor communicably coupled to the OS, the hypervisor having read shadow means for trapping read requests to the MSR, and write mask means for trapping write requests to the MSR; the hypervisor having means for responding to the trapped read requests and trapped write requests, wherein presence of the monitor is obscured; the processor having an instruction set architecture (ISA) including a system return (sysret) instruction configured to effect a return of execution from the OS to an application running on the computer; and sysret monitoring means for intercepting calls to the sysret instruction, for executing sysret monitoring operations, and for subsequently effecting said return of execution.