I accidentally encountered this problem when I checked some ebooks out of the library. I've got Adobe Acrobat 7.0 installed on my computer and, for some reason, Adobe's new Digital Editions software was automatically installed to my computer (without my knowledge). When the Digital Editions software was installed to my computer the file association for PDF files and EBT files was changed to Digital Editions. When I opened up the library books, they opened up in Digital Editions. I couldn't figure out why I could not read them in Acrobat 7.0 or why I could not send the books to my pocket pc. Later, after spending quite a lot of time looking for answers on the internet, I figured out what happened.
This is what you need to do:
If you installed or upgraded to Adobe Acrobat 8.0 or Adobe Reader 8.0, uninstall the software and go back to Adobe Acrobat 7.0 or Adobe Reader 7.0. You can still download this version of Adobe Reader from Adobe's website.
Then uninstall the Digital Editions software completely from your computer.
Open Adobe Acrobat 7.0 or Adobe Reader 7.0 and disable the feature that automatically updates the software (to prevent your software from being upgraded to 8.0 without your knowledge):
Edit > Preferences > Updates > select the option "Do not automatically check for critical updates"
Change the file association for PDF files back to Adobe Reader 7.0 (or Adobe Acrobat 7.0): open up File Explorer > select Tools > Folder Options > File Types > find and highlight the PDF file type > select the "Change" button and select Adobe Reader 7.0 or Adobe Acrobat 7.0.
This won't fix the books you've already opened in the Digital Editions software, but now when you download DRM protected PDF files, you can use them as you are accustomed to.