The vast majority of Windows exploits are not caused by bugs.
They are exploits of Active X, which by default, allows a website to install and run software on a user's computer.
Microsoft has finally become more security conscious, but they dropped the ball for years and their customers are paying the price. Apple needs to be very aggressive in squashing bugs and security holes to ensure that they continue to be secure, but hackers will still need to find bugs to exploit rather than disasterous design decisions.
Bugs ARE bad design decisions.