Macs don't run things, PC's do.
Macs are expensive. PC's aren't.
If you know what you're doing, a virus on a PC isn't a big deal. Can't speak for Mac though, this may be true for both.
If Macs were popular, viruses would actually be designed for them.
Vista is a good operating system if you A. Have a good computer, or B. Know how to Google things and disable the special features.
I haven't had my PC crash because of Windows since Windows 98. In fact, in using Ubuntu in my Computer Science class, it crashed multiple times. The claim that Windows crashes a lot, frankly, is old had.
Microsoft actually innovates - for the most part everyone else copies them and makes minor improvements. I challenge anyone to find a program more useful than Excel, with the exception of a browser.
I'm done, but I think I've made my point clear. If you want more reasons, ask me later.