In re: Linux & Viruses becoming a target... Microsoft is making moves to compartmentalize the OS like linux has, where you MUST escalate privileges in order to do things like install executables, etc. It's a step in the right direction, but they are nowhere near where they need to be. Microsoft is a target because they are SO easy to maliciously code against.
If you want to take that to the next level: Compare Apache vs IIS. Apache is by far the de-facto standard for webservers on the internet, and, regardless of whatever MS sponsored marketing hype tells you, holds dominant market share over IIS. using your reasoning, exploits against apache should be a dime a dozen. Are there? um, not last time I checked.
Proof, apache vs IIS: http://news.netcraft.com/archives/web_server_survey.html
scroll about 1/2 way down.
This is where linux (and to a MUCH lesser extent, Apple's OSX) has a significant lead. In order to make system changes, install binaries to protected locations (other than in your own /home/$user/bin/) and make changes to system configuration, you MUST be the root user. Granular premissions settings on system files, within /etc/ and other locations, completely prevent unathorized alteration. This is a concept that many M$ users can't grasp.
Another problem with MS OS's is the lack of the idea of protected system memory. Many programs can (are allowed to) access blocks of memory. This leaves the system vulnerable to remote exploitation, such as buffer overflows and compromise by a malicious local user (or local program). Remember, Windows was never built from the ground up to be a NETWORK operating system. As networks grew and the demand for interoperability increased, microsoft released their completely busticated TCP/IP stack, and the marginally stable OS that stood behind it. In addition, the security model was 'hacked' from a weak local one, to an even weaker network one.
Where does apple fit into all of this? Well, darwin (Mac OS X) is so bastardized by apple that it barely resembles the UNIX platform that it was based on. Besides, I personally think Jobs is a greedy prick, so my opinion is biased there.
My windows vs linux comparison comes from having over 10 years of experience in the IT field, as an administrator of both OS'es in enterprise environments. Trust me, I'll take linux over windows anyday, but darn.. Windows sure can pay the bills sometimes, cause it needs a lot of babysitting.