Resolved Excel sum not working

I was recently upgraded to Excel 2007. I'm working with a file that was created in Excel 2003. When I clear all of the numbers in a column the column sum does not change. I checked to make sure the sum calculation was correct and that all of the fields were numeric. The only way that I can get the sum to recalculate is if I hit F2 and then enter. But I have to do this every time I change a number in the column. Why won't it automatically recalculate the sum? Please help.

 

Thank you for the response Steve. I familiarized myself more with Excel 2007 and found that 'Manual' was checked under the Calculation Options. I changed it to 'Automatic' and it's working fine now. Thanks!