A recent study by ZDnet UK showed that in 2007 alone, Windows Vista and XP had a combined 23 vulnerabilities reported throughout the year. However Mac OSX had 234 highly critical vulnerabilities reported. This means that in any given month alone, OSX has five times more vulnerabilities per month than both the Windows versions combined.
I can only imagine what apple is thinking about this now. But this only makes Windows Vista and XP shine even more in light of the recent backlash that Vista has been weathering for the last year since it RTMed.