Writing for Security Focus, Scott Granneman explains why it's not just the huge number of computers that make Windows virus-prone, it's the operating system's architecture:
Security is, as we all know, a process, not a product. So when you use Linux, you're not using a perfectly safe OS. There is no such thing. But Linux and Mac OS X establish a more secure footing than Microsoft Windows, one that makes it far harder for viruses to take hold in the first place, but if one does take hold, harder to damage the system, but if one succeeds in damaging the system, harder to spread to other machines and repeat the process. When it comes to email-borne viruses and worms, Linux may not be completely immune - after all, nothing is immune to human gullibility and stupidity - but it is much more resistant. To mess up a Linux box, you need to work at it; to mess up your Windows box, you just need to work on it.
[Source: Verbatim from Macintouch - the Macintosh technical resource]