Excel 2000 and Excel97 compatibility error???

Execl 2000 and Excel 97 are installed on different
workstations in my office.

Sometimes files created on Excel 2000 don't open from
Excel 2000 and shut with the error message below.
"EXCEL.exe has generated erros and will be closed by
Windows. You will need to restart the program."

They will open on Excel 97 without any problem.
Also after saved on Excel 97, it will open on Excel 2000.
Once saved with Excel 2000, it won't open and cause Excel
to shut.

The files are saved "Microsoft Excxel 97-2000 & 5.0/95
Workbook" format instead of "Microsoft Excel Workbook".

Does anybody know what cause this error and know the
solution?

Thank you,