Originally Posted by mr ploppy
Yes, shortening the filenames fixed it. I do have quite a long path to where the ebooks are, I forgot that would affect it too. But the file was in the same place, with the long names, before I opened it in Sigil and it opened in Calibre's viewer OK then. It was only after opening it in Sigil that it stopped working.
really weird. EPUB is a Zip file without the full path to the original source folder.
BTW XP SP3 here, also.