well, that certainly just is not true. I got a chance to learn a SMALL amount about this stuff last semester. My professor happens to be the lead release engineer for the FREEBSD project. The guy knows Operating Systems better then most programmers, Admins, etc… The way he explained it was that XP does a lot of letting third party software access hardware directly(like 3d games and shit). They have been cutting down on this sort of thing with Vista. It is the reason why a lot of games that run on XP don’t run on Vista. As well as the human engineering aspect of everyone running around on their machines as Admin, and just clicking the hell out of things.
With a Unix based Operating System, you can do ANYTHING once you become the root user. So most all unix exploits are trying to find a way to become root user and then just execute commands as root. That is how the exploits are done on unix OSs(Linuxes, Mac(Darwin), BSD flavors)
With windows, its just so much easier because 99% or users are always logged in as Admin, all the work has been done for them. The only thing virus writers need to do is convince the user to click the icon.