First, I'd like to echo some of the other people in saying thank you for Calibre - it is one of the best and most useful programs I've ever used, and even better, it's free and open-source!
Like at least one other person who posted, after uninstalling the 32-bit version of Calibre and installing the 64-bit version, I was unable to set ebook-viewer.exe as a default program for any file type. In the selection dialogue, I would select the program and it would not appear on the list. After reading this thread, I searched through my registry and found several remaining entries referring to the old Calibre directory in C:\Program Files (x86)\. After I deleted all of these entries from the registry, I was then able to set the newly installed ebook viewer in the C:\Program Files\Calibre2\ directory as a default program like normal.
I have no idea why it wouldn't let me do that in the first place - perhaps there's some kind of bug in Windows 7 having to do with programs with the same file name in different directories? - but in any case, I was able to get it working.