Sadly, I don't think such an app exists. I've been looking ever since I got a Win7 tablet.
Blio sounded like it was going to be it, but as you discovered, epub support in nearly non-existant (it will download epub from feedbooks, then convert it to XPS). Even if you convert them yourself to XPS, it's still cludgy and buggy.
Hopefully someone will come up with something, as ebooks start to take off.