The problem isn't Windows, it's users that are willing to run free-porn.exe that is linked in facebook/email/whatever.

Any operating system is only as secure as the user operating it.

A properly configured Windows 7 machine with a solid antivirus, firewall, and a user who paid attention during 15-20 minutes of information assurance training would be a real bitch to exploit.

Even if the odds of a match being 5 billion to 1 in a population of 214,597 there is a 99% chance that one person will have a match with another person.

See the wikipedia article on the http://en.wikipedia.org/wiki/Birthday_problem Birthday problem to check the math.