Thank you for your reply but I do not understand why Adobe will not let you delete your own ID.
However, I did what I mentioned in my original post and installed ADE 3 on my laptop. A box appeared asking me for the vendor (of the ebook I presumed) and the vendors login and password. After several unsuccessful attempts at using my library details, I used exactly the same Adobe details to authorise the laptop that I had used for the desktop. Instant success, so I will just use the laptop from now on as I cannot be bothered to spend any more time trying to find out why the desktop would not work. The ebook has now been successfully transferred to my Kobo Touch in EPUB form and I am enjoying reading it.