Groovy images on a zdnet blog that purports to demonstrate that Windows is "inherently" harder to secure than Linux. That may or may not true but I don't think the diagrams show what the author thinks they do. It is entirely possible they just show greater modularity in Windows.
Either way, the pictures are interesting and it could be a useful way to reason about such things if applied carefully.
[EDIT 2006-04-22. Corrected typo. Thanks for spotting it, Dave]