Can't open books that used to open perfectly!

I've been using Calibre for some time. Then this morning it wouldn't open a book as expected. After some fiddling about I completely uninstalled it, reinstalled the latest version, and installed my "Apprentice Alf" DeDRM tools.

It looked fine, the plug-ins said they were installed etc.
The only problem: it won't even open the books in my library, and this includes those without DRM

Here is the log file from trying to open a non-DRM mobi file -

calibre, version 0.9.33
ERROR: Could not open ebook: [Error 5] Access is denied: u’C:\\Users\\Doug\\AppData\\Local\\Temp\\calibre_p lbykg\\oeosko_ebook_iter’

Traceback (most recent call last):
File “site-packages\calibre\gui2\viewer\main.py”, line 40, in run
File “threading.py”, line 763, in run
File “site-packages\calibre\ebooks\oeb\iterator\book.py”, line 99, in __enter__
File “site-packages\calibre\customize\conversion.py”, line 234, in __call__
File “site-packages\calibre\__init__.py”, line 441, in __enter__
WindowsError: [Error 5] Access is denied: u’C:\\Users\\Doug\\AppData\\Local\\Temp\\calibre_p lbykg\\oeosko_ebook_iter’

Any suggestions as to a possible solution? Or have I just forgotten to do something basic? All suggestions gratefully received.

I run Windows 7 Professional, SP1, and Norton 360 Premier Edition.