Suddenly, EVERY spreadsheet I open and work on can NOT be saved without
getting an error message "Your changes could not be saved to '{file
name.xls}' because of a sharing violation. Try saving to a different file."

If I do that, it saves to a file such a C867893 or something like that
(letters and numbers vary). If I then try to Save As and use the old file
name I get the message "The file '{filename.xls}' may have been changed by
another user since you last saved it. In that case what do you want to do?"
I am then given two radio button options - Save a copy (default) or Overwrite
the changes.

If I choose to overwrite, I get a third error message "Document not saved".

This is happening to ALL of my spreadsheets and has only just started
happening! I do NOT have these spreadsheets shared not have they ever been
shared.

Any idea of what is happening, why it is happening and most importantly, HOW
DO I STOP THIS from happening - it is VERY annoying and time consuming.