Kovid, something broke after 0.6.18
Never before did upgrading to the latest Calibre break the file associations.
I am running Windows 7 and after I upgraded to 0.6.18 and 0.6.19, it was a major pain in the butt to reset the .epub association back to the reader app.
An easy way:
Pin the ereader to the taskbar.
drag an .epub to the ereader icon in the taskbar.
You will get a "open with" dialog, for reasons that I don't want to understand, this particular association dialog works, when all the usual ones do not.
I bugged it.