The chart shown compares security measures in-built into various operating systems. Blue is great, gray is good, white is bad. The operating systems are listed across the top; the security measures being assessed are down the side. These are all measures which have made their way from academic think-tanks into real-world code only during the last few years. More details here. Windows as the most widely used of the operating systems out there, has been the biggest target for hackers, and therefore OS X or RedHat LInux may be less of a threat in some sense. And to be fair, the chart is a bit behind on the features offered by OS X, which will undoubtedly improve in the next release. Still, the chart clearly shows that anybody with the hardware to upgrade to Vista from XP should do so quam primum.