"There are something like 50,000 known viruses out there in the windows world, and this in my view has something to do with all the little companies and products which msoft has swallowed up and spat out the bones thereof over the years; I would GUESS that a lot of the people who used to work at those companies now sit around writing windows viruses.
There are something like five or ten known viruses in the linux world and they're all targeted at servers, the chance of me ever getting one of them is basically zero."
By the latest McAfee anti-virus listing there are just under 90,000 viruses for Windows based PCs. They are not being written by ex-employees, the majority are written by young guys with programming skills (eg teenagers and Uni students), the same group of people who are into cracking games, hacking systems and generally testing themselves to see how far they can get.
Linux is not a more secure system, its simply a less popular system and is therefore less targetted. Virus writers want it to spread and be famous, to do that you aim for the biggest group you can, hence all new viruses are being aimed towards Windows XP machines, with many of the new viruses not affecting older Windows versions like 95, 98 and Me.