By default, Excel AutoCalculate will sum the numbers.

Excel 2003

You can change the calculation by right-clicking on the Excel AutoCalculate area and choosing the desired function.

Excel 2007

1.  Highlight the range to calculate.
   
2. The Autocalculate feature will show calculations in the Status Bar.
   
   
   
3. To add or change the type of calculation, right click on the Status Bar Autocalculate area.