Andres Guadamuz of TechnoLlama has an interesting theory for the reason so many Windows computers are vulnerable to attack. (The primary reason is that people don't update their computers. Each time I see my dad, or when my daughters come home from university, I spend time updating security items on their computers.)
In New Windows virus infects millions of computers, he has a second reason. He blames Microsoft itself for creating distrust in its customers. Recall that Microsoft promised to never update Windows without your permission, but then did it anyhow, secretly. The update negatively affected honest users when its newer version of its bootleg software checker (named "Windows Genuine Advantage" in a 1984-ian way) falsely determined that numerous installations of Windows were not legit. In the case of Mr Guadamuz, he bought had a retail copy of Windows to run on his Mac.
When Microsoft broke its promise by updating Windows in secret, Mr Guadamuz figures some customers lost sufficient trust in the convicted monopolist that they no longer allow any more updates, valid or not.
Of course, Microsoft has been in the business of breaking its word to customers and "partners" for many years, such as when they pretend to be interested in an acquisition only to steal the intellectual property. Each new version of Windows makes extravagant claims of making the life of users better, but then features like ActiveX and embedded VBA macros make life worse due to the influx of malware for which customers are not compensated.