We utilize a multi-page workbook in Excel 2003. All updates are installed.
Recently, random formulas are not yielding a result, rather show a default -
for 'zero'. Not all formulas are the same type, e.g. round or sum, nor will
they necessarily repeat. Calculation is set to Automatic, no files have
Calculation set to Manual, no macros are used. F6 is depressed to refresh
formulas, but yields no result. Nor does F8.

Their are two methods we have found to yield a result: 1) edit the formula,
by replacing at least one character with the same character - generally we
replace the = sign with itself, depress Enter and the formula yields a
result; 2) right click the worksheet tab, copy to another sheet, and ALL
formulas yield results.