This is directly the result of very insignificant quantities of highly damaging mass-spreading malware being written for other computing environments like Linux, BSD and Mac OS X.
Interesting, huh? See, I knew those hackers were smart. I knew they spent more time on Windows because the impact was greater.
Of course, if you notice in my previous post, Linux bears the brunt of most direct hacker attacks (and gets eaten alive by them). Windows has such a huge user group, and has such a wide range of users, that the target is greater. Plus, no other OS is as widely connected as the Windows OS. It's not as easy to attack the other OS's because they're so strung out, unlike Windows which is practically everywhere in the world, chained from business to business through 24/7 emails, servers, and other networking/business-related processes.
Securing an OS distributed and used on the scale of Windows is a monumental task. I say it again: "It's amazing Windows functions at all!" A true testament to those underpaid, underappriciated programmers.