Worksheet/File Coruption
 

I have an Excel workbook with multiple worksheets that are updated each
month. I copy the previous month's file and then update. When I save and
then reopen, the file has errors and cannot be read. I've tried copying the
worksheets individually into a new book and this works for a short period of
time and then again the file cannot be read. I'm using Excel 2003 and have
downloaded all service packs. Is there something I should be looking at or
am doing incorrectly?