I wrote:
I suspect that either or both E1 and F1 had a Text format
before you entered their values, and you enter their values
in the form =2. At least, that is the only way I can get a
#VALUE error when E1 and F1 do not themselves have errors

I should mention that I am using Office Excel 2003. It is possible
that earlier revisions of Excel is less tolerate of other text forms
of numbers. So it might be easier for you to cause the #VALUE
due to cell format mismatch and/or typing errors.

Bottom line: explicitly change the cell formats for E1 and F1
to General or Number, re-enter the values in E1 and F1, say
"abracadabra" :-), and see if your problem goes away.