That study was just about security though, not stability. I'm sure OSX is fairly stable since it runs on unix these days. But at the same time, XP rarely ever crashes on me either, which has pretty much been the case for Windows in general ever since they put the true 32-bit WinNT/2000 core in their average consumer desktop OS's (starting with XP). There's really no reason for any computer running any modern operating system to outright crash, aside from faulty apps (usually third party, and ones that interact with the system at a lower level) and faulty drivers (again, third party). That's the only things that's ever caused mine to crash. Third party apps make a product look bad when they make it break, which is partially why Windows has such a high public perception of instability (more third-party apps to crash it), while Mac has more of an image of stability (most software people run is from Apple themselves, then immediately followed by big boys like Adobe). It's part of the reason they won't give an SDK for the iPhone, because they don't want its image to be tarnished with possibly poorly coded software.
Anyhoo, as for security, it's no surprise that Vista would take the lead, considering the entire OS was restructured with a security mindset. Mac was after that (arguably from its small marketshare), then XP. All fairly expected in such a comparison I reckon. But then comes the funny part, with Linux trailing behind those, especially Red Hat Enterprise Linux (a pay version for companies!), despite all those claims that Linux was the most secure.
