I didn't really read through this thread much before my first response, but after reading a little bit more I have some things to add.
Windows, provided the applications and drivers being used are stable, is a pretty competent desktop computing platform. It has a ton of applications available and can use a wider range of hardware than any other OS out there. If properly administered a Windows box can remain functional for quite a long time and not suffer from viruses or spyware or other maladies. I have ran every consumer version of Windows starting with Windows 98 SE and have never had a virus.
What caused me to switch from Windows was not security, but simply a buggy NVIDIA SATA driver in Windows Vista that prevented me from using iTunes and QuickTime. Given that I don't feel like trusting a buggy driver for something as essential as my hard drive data and really liked my newly acquired iPod touch, I knew I had to do something. XP would not recognize my onboard sound and I could not get any driver to work for it so that was not an option. My old iBook did what I needed except for gaming, and installing Ubuntu on my desktop (replacing Vista) allowed me to run the games I wanted to (mainly World of Warcraft). As far as security goes, that was a complete non-issue in my decision to switch. I'm platform agnostic anymore and care far more about what I can do on an OS than anything else. For a long time Windows served my primary needs best and now Mac OS X does. In the future something else will replace OS X.
Windows Vista, Mac OS X, and Linux all have major security vulnerabilities due to what is between the keyboard and the chair. Granted administrator access, a bad app can wreak havoc on any system. Although it doesn't seem to have been done yet, it is perfectly possible for OS X and Linux to run spyware or get infected by some other malware. All a user has to do is click "Grant Access" to some popup dialog box and allow an app to run with full privileges. Windows did make it easier to allow malware to work on a system prior to User Account Control in Windows Vista but even so, just a little caution went a long way with XP and prior.