I'm not sure where on Windows the temp folder is, but IIRC the temp folder will be cleared if you reboot (and possibly just by re-starting Calibre). I seem to recall some scripts in another thread to clear the folder automatically, but you might need to do some digging with the search function.
Edit, here you go:
https://www.mobileread.com/forums/sho...d.php?t=124145