ok I need help on this question cause I am learning the computer, so I am try to understand the question.
closed as not a real question by SvW, MDMarra, joeqwerty, Lucas Kauffman, Jim B Mar 18 '12 at 13:46
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They aren't more or less vulnerable to exploits, they just cover a bigger amount of targets than OSX and Linux.
Well, looking back in history, Microsoft Windows operation systems have been the most used ones in the home computing area ("desktop PCs"). Therefore millions of computers are running Windows, and every Windows version contained various security vulnerabilities.
In addition, millions of software vendors have been writing software for Microsoft operating systems, and many of them also contain security issues.
Some other issues occur through obvious misconfiguration of Windows systems, such as using Windows as an admin user or leaving password fields blank.
Since Windows is spread so widely, there is a huge interest for attackers (criminals, kiddies, curious ones etc.) to write software which is able to exploit configuration or security weaknesses. Those tools/exploits can often be used by people who do not posses good security skills, therefore a Windows system can sometimes be "cracked" without much security knowledge.
When we look at Linux and MacOS at the other side, they seem to be more complicated to the common user. Furthermore, those systems are not as widely spread as Windows systems on home PCs. Because of the concept of *nix based systems, a normal logged in user does not work with root privileges. Therefore injecting malware to the desktop session is more difficult than on Windows systems.
One could argue that if Linux and MacOS were as popular on desktop machines as Windows, there would be more malware for those systems and Windows would loose it's reputation as being "insecure".
However, Windows server distributions, being fully patched and equipped with a proper firewall and AV software, still make it hard for attackers to break in.
In most cases, it is the fault of poorly written software that attackers succeed, no matter if it's an underlying Windows or *nix system.