Windows was built over the foundation of the DOS disk operating system that shipped with the very first PCs from IBM in the early 1980s. This brought with it several problems, many of which still exist today. The biggest issue is the need to maintain compatibility with legacy hardware and software. DOS did not support or need to support multiple users or multitasking. Support for these has been added with later versions of Windows.
As computers have changed in the last 30 years, and with the introduction of new technologies such as the Internet, the need for extra security has come to the forefront of operating system design. Unfortunately, this has meant having to build security over the top of the existing Windows system. This has inevitably led to some compromises and security flaws, which have been exploited by the authors of malicious software. With Windows 8, it is rumored that the legacy support will be moved into a virtual machine. This means that the security subsystem in Windows 8 will be able to be treated differently, making it much more secure.
Most other desktop and server operating systems, including Linux, Apple OS X, and Google Chrome OS, are all built on top of an operating system called UNIX. This operating system was developed in 1969 and was initially designed to accommodate multitasking and multiusers on mainframe computers.
This means that user permissions and overall operating system security have always been handled differently in UNIX, with users never being given default administrator access to the operating system files. UNIX has slowly made its way from mainframe and minicomputers over the years to the desktop market, during which time this security system has remained unchanged.
None of this means that Windows 7 is an insecure and unstable operating system, quite the opposite. It is the most secure and stable operating system that Microsoft has ever released, and many experts believe it to be every bit as secure as a UNIX-based operating system. It is the vast popularity of Windows that has made it such a security target in the past.
Source of Information : Microsoft Press - Troubleshooting Windows 7 Inside Out