FBReader is fine for most ebook formats, including epub. However, it doesn't support all the features, like CSS. I use it myself and for general reading, it does the job.
You might also install the Sony Reader Library software, even if you don't have a Sony ereader. This software supports epub quite well. The reading mode does a very good emulation of what you would see on the screen of the physical ereader. I particularly like the fullscreen mode that blacks out the Windows desktop.
Joe
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