Excel spreadsheet errors

As a finance guy, my boss works primarily on Excel spreadsheets and quite a few of them are linked together via the formulas. However, he has been receiving errors on the sheets and the only way to get rid of them is to exit Excel and start again. First of all, the numbers in some cells will appear as ##### (and it is not a problem with the cell width as I have checked) and the font in the other cells will change. Could it be that the file is corrupt? If so, how do I fix the problem without having to re-link the files again?