I just purchased Ashampoo Office 2010, and am testing it. I created a simple spreadsheet, merging the top two rows of cells (columns A through M) and typed the sentence:
Now is the time for all good men to come to the aid of their country.
in the cells and save it as a Microsoft Excel 97/2000/XP/2003 xls file. I then tried opening it in Microsoft's Excel Viewer 97 and it crashes. Works fine in Microsoft Excel viewer 2003. If I open in OpenOffice.org 3.3 and re-save it works fine with both versions of the viewer.
I normally use OpenOffice.org and test files in this manner before sending to others. If necessary, I can upload the file.