They are not actually corrupted. CF breaks the secure open feature like i said so they appear as corrupted. If you right click, properties, and tick the unblock box they will work.My excel's are randomly corrupted and won't open. Secure open needs certain files, locations, etc to determine if they will start with it so that is why you think it's random. I need to send them to somebody over Skype and have them sent back, then they open
There is an even easier solution which is uninstalling the bug magnet called comodo because this bug is one of many.
Example to test. Grab this random doc from the internet.