Are you sure you're saving in an Excel 97 compatible format?
Excel 95 had a 255 character limit for any cell contents. The limit for Excel 97 was much larger, although the limit of what could be displayed in a cell was 1024 characters..
Anyway it seems likely that you have cells which contain more text than is allowed in the version you are saving to.
Obviously you can go through the cells and find those that exceed the 255 character limit, but if it is necessary to use the workbook in an earlier version, there have been suggestions for workarounds, such as saving the text in a text box. You might be able to use adjacent cells.
In your current version (Excel 2003) you could use data validation on the cells in question and limit data entry to a maximum of 255 characters, to force compatibility. (Data -Validation-Settings-Allow-Text Length and set Min/Max values to 0 and 255