InfoWorld has a very good post on Microsoft's recent security advances. Microsoft has spent billions on security and now it is beginning to show. It's not that security is extremely difficult on its own; rather, it is the combination of security and compatibility with old software and old hardware that is the trick. Apple has been able to switch operating systems and start all over again. DOS application, on the other hand, still run on Windows. Personally, I prefer Apple's model but I can understand why Microsoft felt it had to drag its history into the future.
The posting covers a number of the new techniques that closes the doors on the most common avenues for attacks. Whether we like Windows or not, I think we can all agree that reducing the number of zombie computers slinging spam by the billions is a highly desirable goal.
What does all this mean to you? Basically, the answer to whether Windows is security or not must now be considered seriously and not just addressed by a derisive laugh.