Dino Dai Zovi, the guy who won the $10,000 MacBook Pro hijacking contest, claims that Microsoft's Windows Vista operating system is safer than Mac OS:
From your research on both platforms, is there a winner between Mac OS X 10.4 and Vista on security?
I have found the code quality, at least in terms of security, to be much better overall in Vista than Mac OS X 10.4. It is obvious from observing affected components in security patches that Microsoft’s Security Development Lifecycle (SDL) has resulted in fewer vulnerabilities in newly-written code. I hope that more software vendors follow their lead in developing proactive software security development methodologies.