If you believe Jeff Jones, Microsoft's security strategy director, then Vista is much more secure than XP, Mac OS X, and Linux.
Why, oh why, is anyone paying any attention to this man outside of the walls of Microsoft? His paychecks are signed by Bill Gates, people!
What do you think he's going to say? "Really, when you get right down to it Windows is a fatally flawed operating system that no one in their right mind would ever use for truly secure computing?" He wouldn't just be fired; he'd be carried out by the real Microsoft security: the Microsoft cops.
Or, let's say he said something more politically correct, such as, "Really, Windows, and now Vista, still lag behind both Linux and Mac OS X when it comes to security, but we have gotten better." Much nicer, but still true and he'd still end up fired.
Anyone who believes a word out of Jones' mouth when it comes to Vista security is a fool. You want to find someone who does say nice things about Vista security from time to time that you can believe in? I suggest Larry Seltzer's Cheap Hack blog.
Jones also goes on to explain that in the past he's worked with Unix and the BSDs. He's also willing to admit that security improvements are happening on Linux and Unix, but that Microsoft is doing it better. He then asks, "Am I biased? I do not think so, but let's just all keep assuming I am, because I don't mind. If I make comparisons, I'll lay out my metrics."