I have some additional data points:
1) I created another new spreadsheet, and mysteriously, it was behaving as expected. For example, simple equations like =100+200 indeed showed a value of 300. In my problem spreadsheet, that cell would be 0.
2) This information I think may be material (for those of you who know) - When I cut & paste from the problem spreadsheet to a new spreadsheet file, it only pastes as if I was selecting "Paste Special by Value". All of the formulas disappear. And if I do try "Paste Special by Value" it gives me choices as if I'm pasting the whole file. The first choice is "Microsoft Office Excel Worksheet Object". Excel is acting like the problem file is NOT an Excel file.

Very strange!

QUOTE=MarkBohrer;959641]I am having the same problem as the original post. I have a spreadsheet I've been using for weeks now, adding data each day, and suddenly every calculated cell that I edit (most are filled with formulas like =A1/A2) ends up zero! Even previously working cells, if I hit F2, but don't change anything and just hit enter, then suddenly the cell value is zero!

I tried Dave Peterson's suggestion below, changing these settings, but that didn't fix it. There must be some setting that is doing this! I even created a new empty spreadsheet and get the same behavior.

Here are the contents below of the test spreadsheet, starting in cell A1, with the contents of formulas in cells A3 through A7 shown in text in B3 through B7. This has to be really simple. But what???

100 Content of cells in column A
100 A1 and A2 are 100
0 =A1/A2
0 =100/100
0 =200/100
0 =1
0 =SUM(A1:A2)[/quote]