ADE2.0 is a very buggy program. I still use 1.7.2 as it is adequate for me and works with my PRS-350; but I can see that there are a couple of features of ADE 2.0 that would be useful (if it worked at all).
First, just in case. Did you have an option 'skip' or 'migrate data' when installing ADE2.0. Try choosing the option 'skip'.
(Just did a test and installed V2.0 on Windows 8 using the 'Skip' options, and without un-installing 1.7.2 and it worked.
Of course, that doesn't mean it will work for you ...)
If you have done that or it doesn't work, try the following. I can't promise anything as I haven't tried it; but .....
Uninstall v1.7.2.
Check for all directories relating to Digital Editions and delete them (or safer, rename them in case they contain books etc). eg
C:\Program Files\Adobe\Adobe Digital Editions
C:\ProgramData\Microsoft\Windows\Start Menu\Programs\Adobe\Adobe Digital Editions
C:\Users\<userid>\AppData\Local\Temp\Digital Editions
C:\Users\<userid>\AppData\Roaming\Adobe
C:\Users\<userid>\Documents\My Digital Editions