It depends. Yes I agree windows has a much larger install base and has a lot more 3rd party products that can expose the user to issues. But one of the "selling" points of Macs is it's perceived as more secure than windows. But as proven by things such as the pwn2own contest it's not. So for users looking for a more secure platform they are better off learning secure practices and not relying soley on OS marketing hype.
If you feel that 3rd party apps on windows needs more security attention, have at it and post all day long. I'll focus on the FUD being spred by macbots.
You are talking about an artificial situation that has nothing to do with the way people use computers.
Google now has a policy of not letting people use Windows machines because that is how their network was hacked, through Windows machines. All the programmers were already on Mac on Linux. So this affects the non-technical people at Google. That is a real-world situation.
If you are looking for a secure computer, I have one for you. It's an old Dell laptop running Windows 95. It's secure because it's in my storage room, it's not hooked up to a network, and you will have to get the key to the room from me. At last, we have found a secure Windows machine.