Wow, lots of FUD here, popular thread!
1) Apple computers are more expensive because they are more powerful and have more features.
For example, price up a Dell or Sony or whatever, with the same spec as the current MacBook. The prices will compare pretty evenly (in fact, Apple is quite competitive, especially if you think about the hardware design and Mac OS X).
Apple's philosophy on hardware is that it should be powerful enough to do anything the software is capable of (including video editing with iMovie), so all their stuff is higher-end. PC manufacturers sell the complete spectrum of stuff, including lots of cheap stuff. While it may come with Windows Movie Maker or whatever, it won't be able to run this well. This is just a difference of viewpoint.
Viruses: a virus is just a program that runs by itself. As such, it's trivial to write a virus for any system, including Mac OS X (terminal -> rm -rf / should do it). The question is, how easy is it to get a virus by just running your computer on a network (like the internet).
With the security system in Mac OS X, anything modifying the base software needs a password from the user. So it's typically up to the user to run this virus or not (and thus, the user's intelligence and ability with a computer).
Windows has far more users than Mac OS X, and thus there are far more stupid people using Windows. That, and the higher target that Windows is than Mac OS X for virus writers, is why Windows has more viruses.
Windows' security is in many ways better than Mac OS X (memory randomisation, etc.), but this is required again to it being higher target.