What if somebody wrote a virus that infected windows computers to replace the OS with linux?

That would be a hell of a virus!

But people would notice, and pretty quickly. Even if this megavirus could somehow manage to do it without data loss, mimicking every aspect of windows perfectly, making file structure work the same, etc; it couldn't do anything about the programs that simply don't work on linux yet.

Once the first person noticed, it would be a huge deal

That would be an anti-virus.. x)

Since it would remove spy-ware from your computer.

Even if it were possible, personally I wouldn't recommend using it as it would violate/contradict the system of trust and autonomy

Forcing the belief of:

• "you have to use Linux because I believe it's good"
  • "because I believe x means you should believe y";

onto someone makes that idea inherently untrustworthy and the foundations of its origin would be unstable.

My recommendation is that if you want people to use Linux or something because you trust it and want others to benefit in the same way; then it's existence and systems contained within it should act as proof itself and shouldn't have to be forced onto others

• similar to opt-ins rather than opt-outs

It would be still a malicious attack, that deletes data and files without the users consent and probably without knowledge at all. And it would make look like Linux is a virus to people. I highly discourage anyone writing such a hostile virus, that is hostile to the user and to the Linux community. Bad idea.