This problem started when I was upgraded to Excel 2003.

When I receive an unprotected Excel spreadsheet from someone else, I am
unable to add a formula that works. For example, I will enter a sum
formula for a column of numbers, and the total will be 0.

I've tried:
* changing the the format of the input cells - back and forth - from text
to numerical, etc.
* turning on and off error correction, and sharing.
* copy, paste special, values.
* saving the worksheet under a new name.
* turning manual calculation on, and off, etc.
* Unchecking link external values under tools, options.

But the only thing that works is if I manually reenter each number in the
column, which defeats the purpose of a spreadsheet.

This is very frustrating.