Can you tell us what the error is (though if it is a typical Adobe Digital Editions (ADE) error message that may not help too much)?
In some cases ADE gets confused and half-authorized.
Try enterint ctrl-shift-D (cmd-shift-D on Mac) when in Library view of ADE, which should clear any such confusion.
Resart ADE, and then try again to authorize.
This program worked just fine on my sons' Windows 7, but he took his computer home. My husband bought me a new Windows 8 to replace my old XP. Now it says I need to authorize my computer to use the program, never had to do this with the computer that I was using. If I could have gotten this program to work on my old XP, I would have been very happy with it. Finally went to bed with a migraine last night. There is no way that I could get it to let me authorize it to down load a book from my library to read.