xls: file format is not valid

Hi,

We have a file server with thousand of excel file on it and every so often there’s an excel file that cannot be open. It’ll get the error “file name.xls: file format is not valid.” I can open it up in Word and all the contents would be there but the all the format will be lost. Does anybody have any idea as to what this means? I’ve check on technet but they don’t have anything on it.