Here was my discovery & solution.
Incidentally, none of the above answers solved my problem.
I even tried using UNLOCKER which proved worthless.
My problem was that of Memeo Autosync Backup
Apparently, this backup process leaves enough of a "ghost like file." This "ghost like file," would show up whenever I would ALT-TAB my computer (Windows Professional XP), i.e. I would see TWO MS Excel Programs running, when I only had ONE visible, on my TASK BAR.
I came across this solution when I thought it might have been the SYMANTEC Endpoint (Anti-Virus) Protection; and disabled the program. However, I kept getting the error message:
cannot delete (LARGE.xls file): It is being used by another person or program. Close any programs that might be using this file and try again.
I subsequently kept seeing the Memeo notice of "syncing" and QUIT the program.
Subsequently, NO ERROR.
For you, it could be ANY of these background saves.