Error message when saving after formatting
 

I have a spreadsheet of about 45mb. My machine is a three year old Dell, low
end for that time. If I make substantial changes, especially formatting, to
the sheet, I get this error message:

"Excel could not save all the data and formatting you recently added..."

After I get this message the first time, it is as if the sheet is infected,
and then it will not save the formatting of even one cell.