Please note that the content of this book primarily consists of articles available from Wikipedia or other free sources online. A virtual machine is defined as a software implementation of a computing environment, which can execute programs just like a physical machine (physical hardware). A virtual machine can provide many advantages, for example, running one operating system emulated within another operating system. Recently, security experts announced the detection of a Trojan nicknamed Crisis, which aims to infect Windows, Mac OS X and VMWare virtual machines and steal data.