Have you tried opening the file and saving it under a new name and then working on that one? Well, actually that is similar to what WWD proposed.
If you have OpenOffice or LibreOffice, too, try opening it with one of those and then save it (using the xls extension, not OpenOffice's ods). If there is any cruft in that file it is possible that OpenOffice just ignores it when saving it.
If that fails - dunno. Write the file from scratch (or tell anyone in IT to do it, whoever is responsible).