You can install v2.0 and v1.7.2 at the same time if you want to.
This occasional inability of Windows to associate filetypes with ADE is a most peculiar and irritating Windows bug.
You could try a third party app such as Types http://izt.name/apps/types/ and see if that helps.
Not so convenient, but helpful for emergency: you can still open .epub files from ADE with ctrl-O, and .acsm files by drag drop from Explorer onto ADE.
There are lots of bugs in ADE2.0. Try replacing ADE2.0 with the older but more reliable v1.7.2.
Version 1.7.2, it is a little difficult to find, available on Adobe site for Windows and for Mac.
The forum software is sometimes corrupting the link above. There shouldn't be a blank in 'editio ns.html'. The following redirects to the same page: http://tinyurl.com/diged172
Thank you so much! The Types app did the trick. I will play around with 2.0 to see how it goes but I appreciate knowing how to get the older version should I need it.
You're the best!
Excellent. For once a bug that isn't an Adobe one.