I have just resolved this issue for someone by opening the file in Excel XP. Here's what caused THEIR error: They'd been copying the same worksheet(s) over and over again into the same workbook. Externally, Excel may name these sheets Sheet1, Sheet2, etc. But internally, a copy of Sheet1 becomes Sheet11, and a copy of Sheet11 becomes Sheet111. Eventually, there are too many characters in the sheet name. Excel XP has a little bit better *error handling* and was able to open the file. The fix is to then save it to Excel 5.0, which doesn't recognize this issue, and the *internal* sheet names revert back to appropriate sheet names. The file can then be opened and saved as an Excel 2000 file again. I cannot recreate the issue in Excel 2000 so perhaps it was something that occurred in Excel 97. I do know that this occurs often with files that have been around for a while, i.e., several years. I'll be happy to attempt the fix with your file too.
~ Dreamboat@www.TheWordExpert.com