Windows is more susceptible to virus and other attacks because of the protection mechanism in it.Given a mechanism there can be many policies which can be used by the virus coding geeks.......The mechanism is such that any kind of memory access is possible through windows interface... Take example of DOS,
it doesnot have two modes of operations: user mode and supervisor mode leading it to have only one mode both for the operating system an the user It means user can gain the previlege rights of the os to perform whatever he want manipulating it at his will......where as linux os has two modes of operations and is less susceptible to virus attacks.............