The idea that PCs get more viruses than Macs is ludicrus. Yes, Windows has some security holes that have been exploited by worms like Blaster...but a good majority of of virus infections are a result of the user opening a file or going to an unsavory part of the internet.
Which...by the way...can happen to a user on any computer that isn't properly secured.
Now..you may think I'm nuts for saying this, but Microsoft has designed their operating system to allow users to run without administrative rights. That many users don't is a result of some software companies requiring administrator rights to run some programs because they either write to the system registry, aren't configured for multi-user environments, or need direct access to the hardware.