A picture speaks a thousand words. This article has 2 images that describe “a complete map of the system calls that occur when a web server serves up a single page of html with a single picture – the same page and picture”.
Just compare the two images, and you will instantly understand why GNU/linux/apache is more secure than windows/iis. You will also understand why GNU/linux is faster and more stable than windows – it’s just better engineered.
The microsoft/iis image really reminds me of … spaghetti code.