Could be a virus on your computer or in the Excel files.

"amted88" wrote:

I have Office 2003, this started a couple of weeks ago that simple formuls
like Sum or =A1+A2, etc, will not calculate. These are worksheets that work
when I have them open and then a little while later will not work when I open
them again. I can close and not save and then try to open them again and the
formulas will work the next time (sometimes I have to open and close it 4 or
5 times before it works).

It seems to be happening more frequently now.

I checked that autocalculate is checked. Even hitting F9 will not work.