In this thesis, we address the problem faced by modern operating systems due to the exploitation of Hardware-Assisted Full-Virtualization technology by attackers. Virtualization technology has been of growing importance these days. With the help of such a technology, multiple operating systems can be run on a single piece of hardware, with little or no modification to the operating system. Both Intel and AMD have contributed to x86 full-virtualization through their respective instruction set architectures. Hardware virtualization extensions can be found in almost all x86 processors these days. Hardware virtualization technologies have opened a whole new frontier for a new kind of attack. A system hacker can abuse hardware virualization technology to gain control over an operating system on-the-fly (i.e., without a system restart) by installing a thin Virtual Machine Monitor (VMM) below the native operating system. Such a VMM based malware is termed a Hardware-Assisted Virtual Machine (HVM) rootkit. We discuss the technique used by a rootkit named Blue Pill to subvert the Windows Vista operating system by exploiting the AMD-V (codenamed "Pacifica") virtualization extensions. HVM rootkits do not hook any operating system code or data regions; hence detecting the existence of such malware using conventional techniques becomes extremely difficult. This thesis discusses existing methods to detect such rootkits and their inefficiencies. In this work, we implement a proof-of-concept HVM rootkit using Intel-VT hardware virtualization technology and also discuss how such an attack can be defended against by using an autonomic architecture called SHARK, which was proposed by Vikas et al., in MICRO 2008.