I read this report about a study into OS security and unsurprisingly in my opinion Linux didnt fair to well.
Around the middle it talks about "security through obscurity" which means that an OS will be more secure because less people use it and so people are less like to make viruses for it.
This isnt taken into account for this study so its amazing that Linux did so badly and Windows did slightly better than some might first expect.
http://www.computerworld.com/securitytopics/secur...0801,97157,00.html