authorizing a new computer

Sometimes ADE gets its registration/activation confused and in a semi-authorized state.

Unfortunately, it often then gives misleading error messages about what is wrong.

A common incorrect message informs you that the ID is already in use on another computer and cannot be reused.

This can often be resolved by completely removing any authorization using ctrl-shift-D to the Library screen on ADE (cmd-shift-D if on Mac).

Restart ADE, and then reauthorize with your (old) Adobe ID.

Moved to Adobe Digital Editions

I think that your question is more related with Digital Editions:

http://forums.adobe.com/community/adobe_digital_editions

This is the Reader forum.