Bit Rot
Several years ago, John Gruber suggested one reason for the virus gap. Namely, that PC users are used to their computers loaded with adware, the sorts of things pre-installed on consumer-level Dells, HPs, and Sonys when they buy them. And so when malware arrives, it takes strong root, and isn't hunted down.
This is a very important difference, one thing that needs to be addressed before Windows becomes truely secure. But for better or worse, it won't. Simply clean installs won't solve the issue. A Mac user, or at least a large portion of them, expect their Mac to perform, to perform right, to 'just work.' For the PC, however, I've come to a theory, a theory which is great flame-bait, but an interesting theory nonetheless.
I think most PC users expect their computers to fail, and hold them to no standards of working. Not only that, but they view the PC as a machine, one that's supposed to break down.
