Try setting the environment "CALIBRE_TEMP_DIR" -- occasionally people report similar problems which are due to the permissions on the TEMP folder. If you set a calibre temp folder, say, in the parent folder of your library folder, this should get resolved.
See:
http://manual.calibre-ebook.com/cust...ment-variables
Alternatively -- your antivirus might be messing around with calibre's attempts to write temporary files. Make sure calibre is excluded from your antivirus's real-time scanning.