Formulas not evaluated

Using Excel 2003, I sometimes get a cell (or a range of cells) into a state where a simple formula is not evaluated but displayed in the cell - e.g. =A1

I can't find a reliable way to correct this except to copy some cell that still works on top of the "bad" cell. Anyone now why this happens and how to recover?

Thanks, Chris

Usually I see this when a cell has been formatted as text first, then the formula was entered.

To fix it, I change the cell formatting to General (or some number type of formatting), then edit the formula ("F2", then "enter" is all I usually do).