Also, the web isn't composed of windows machines. It's a mishmash of a billion different devices. The vast majority of which, know html, javascript, and not much else. Seems a bad idea to only write for a small subset of them.
I don't see where you (necessarily) get "small subset" from.
I do. The less "installed" software I have to go wrong, the better.
I suppose I can always go back to Lynx if these things really bother me too much.
Silverlight is actually pretty good; it's a much easier experience for developers than Flash/Flex. You might hate Microsoft, but you shouldn't hate the tech.
Adobe, while not perfect itself, has been committed to keeping Flash cross platform. This makes Flash harder to hate than Silverlight despite the high cost of developer tools
That will be most everyone. People will do just about anything you ask them to. The prevalence of phishing has proven that. They'll put their online banking creds into any site that has bankofamerica somewhere in the domain name. They'll buy Viagra from an anonymous email through a Costa Rican pharmacy. They'll most certainly click the button that says "Install Silverlight from Microsoft to watch the video".