Slow Excel on new workbook

I have Mac Excel 2008 and using an iMac 10.6.8. I have set up a new workbook last night. When I came back this morning to work on it, it was slow with that beachball going on every move. I thought it was Excel or my computer. I went to set up a new workbook this morning and everything worked fine. But going back to my original workbook...slow. On the original there is nothing in there as to content except the headers and my column set up. Any ideas why this particular workbook is so slow? Thanks.