You need to fix the file associations. Select some random epub file, right click on it, select "Open with...", pick the program you want to use to open the file, make sure the "always use this program" (or something to that effect) box is ticked and just hit okay.
I've been downloading EPub books, from my library, to Adobe Digital Editions, then moving it to my e-reader. But, when I tried to download an EPub book, today, the Windows Media site came up. How do I tell my computer I want Adobe Digital Editions to be the default? This has happened, once before, and the library tech fixed it. She isn't there, anymore and their tech doesn't know how to fix the problem. Thanks for any help.