FBReader is available for windows, if you're looking for open source (keeping in mind that the Android version is no longer open source, so it's not the same FBReader). CoolReader is also available for windows.

Personally, I actually prefer calibre's ebook reader; if you dislike it, but do use a Calibre library, Calibre Web [0] has a (IMO) fairly decent browser-based ebook reader

[0]: https://github.com/janeczku/calibre-web