"Windows is a big target because it powers the vast majority of the worlds desktop computers and laptops. If youre writing malware and you want to infect average computers users perhaps you want to install a key logger on their systems and steal their credit card numbers and other financial data you would target Windows because thats where the most users are.
This is the most common argument for Windows having such a history of malware, and its true"
Maybe both you and your customers might be better off if you just didn't work on Windows systems any more.
But if Windows were ACTUALLY secure, it wouldn’t matter HOW big it was. But it’s not, which is why the various NT Windows-based OSes have had to have hundreds of security patches over the decades.
http://www.pcworld.com/article/173674/adding_up_six_years_of_microsoft_patch_tuesdays.html