I'm no security expert, but I do try to keep up. Someone please correct me if my facts are off. Ahem:
Windows 7 does a lot more randomization of memory layout than Mac OS X does, making it more difficult to exploit executables and libraries shipped with the system. It's actually easier to exploit a Mac OS X machine than it is a Windows 7 machine -- see the results of pretty much every Pwn2Own [1] contest.
That said, the vast majority of spyware is targeted at Windows and doesn't need low-level exploits to do its job. It's the difference between safety and security; you're probably more safe on a Mac, even though they're technically less secure.
Windows 7 does a lot more randomization of memory layout than Mac OS X does, making it more difficult to exploit executables and libraries shipped with the system. It's actually easier to exploit a Mac OS X machine than it is a Windows 7 machine -- see the results of pretty much every Pwn2Own [1] contest.
That said, the vast majority of spyware is targeted at Windows and doesn't need low-level exploits to do its job. It's the difference between safety and security; you're probably more safe on a Mac, even though they're technically less secure.
[1]: http://en.wikipedia.org/wiki/Pwn2Own