I'm am having an issues with Excel 2002 - SP3 (XP)

I occasionally come across an issue where a formula won't display the value.
I.e - cell displays =N3-N4 rather than the answer. Using the watch window I
can see =N3-N4 is displayed in the text column rather than the formula
column. Also the evaluate formula tool tells me that the cell is displaying a
constant.

Please help!

Thanks
Laurence