No offense, but everytime I see posts like this, it's like fingernails on the blackboard to me.
This is such a common myth in the computer world, and it just won't die, even though there is not the slightest bit of truth to it.
Mac/Unix/Linux OSs are architecturally very, very different from Windows. It is the inherent architecture of Windows which makes it susceptible to infection, not it's popularity. The architecture of *nix OSs have their problems, but they are inherently much, much more secure than Windows.
For example, one of the most common ways to infect a Windows PC is via some malware included in an email attachment. To acquire an infection via an email attachment in Linux, the user would have to open the email, then open the attachment. Then he would have to save the attachment under a different name and grant that file permission to run as an executable. Then, he would actually have to launch that file as an executable. In Windows, merely opening the attachment to read it will infect you, since opening attachments in Windows automatically launches them as executable. This remains true of Vista as it was with XP, Win 2K, Win 98, Win 95, etc.
This is just one tiny, tiny, tiny example of the architectural differences between Windows and *nix OSs. And to say that Vista is "more secure" than OS X is patently absurd. I am no Mac fanboi, but *nix architectures (including OS X) are massively more secure than Vista, simply because their architectures were designed from the ground up with securty in mind. Vista was not, as it is a modification of the Win 2003 Server kernel. You can argue that *nix OSs are less convenient to use or poorly supported, but please don't spread nonsense about Vista being "more secure". Despite Vista's improvements over XP, it remains one of the most vulnerable OSs to infection in common use today.