Download Defaults to Acrobat Reader NOT Digital Editions

I got a new e-reader for Christmas and I'm eager to start using it but when I download my library ebooks they are attempting to open in Acrobat Reader not Digital Editions.  Of course, they are not the correct format for AR so I get an error message.  My online library ( www.downloadcentre.library.on.ca ) recommends/supports DE.  I have installed the most current version of it and have it open on my desktop. I've downloaded hundreds of books from the library to DE successfully on other computers, but this is a newly installed operating system (Windows Vista Business) and I can't find anywhere to change the settings to automaticaly download to DE.  Can someone help me to change this setting?