According to research provided by the security firm Securina, Mac OS X has approximately 10 times the number of critical vulnerabilities of Windows XP and Vista combined. We'll let the numbers speak for themselves:
“While Mac OS X had 234 highly critical vulnerabilities reported in 2007, Vista and XP combined had 23, Ou wrote.
"This shows that Apple had more than five times the number of flaws per month than Windows XP and Vista in 2007, and most of these flaws are serious," wrote Ou. "Clearly this goes against conventional wisdom."
Macs have traditionally been viewed as suffering from fewer vulnerabilities than Windows.”
Of course the number of flaws isn't the only important metric here. Often figures like time from discovery to patch, or time from first known exploit reports to discovery are also very important. The good news is that both Vista and Leopard are showing improvements in the area of security compared to their predecessors.