Have you registered your AdobeID with your version of ADE2.0?
If you had a previous version of ADE (even an uninstalled one) it is possible that ADE has got into a confused state with registration.
Try to deregister whatever it has, using ctrl-shift-D (cmd-shift-D on Mac) in Library view.
The restart ADE, and reregister with menu/Library/Authorize Computer.
I have had lots of problems with v2.0, so quite understand you might what to try 1.7.2 again.
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