Screaming Penguin - Source Does Matter

Windows Vista held out longer than Mac OS X, but in the end only Ubuntu stood unscathed. The Mac machine was p0wn3d through a Safari exploit (by making the machine "visit" a web site that could take advantage of the undisclosed vulnerability), and the Microsoft machine, surprisingly, took a Flash exploit to finally crack it.

Flash flaw leads to Vista laptop's fall

Interestingly, despite what Gartner tells us, Linux turned out to be the most secure when actually put to such a real world type test.