I'm using Excel 2002 (on my PC running XP Home)

I'm basically a novice to excel, but can create simple templates for simple
needs. I'm currently creating a template that will simply add the sum of a
certain
number of cells. The problem is, the results don't automatically
"re-calculate".

For example:
In cell E107 I've added the function: =SUM(E6:E104)
The result that came up after I added this function is the correct amount.
The problem is, when I change the amount entered into one of the cells
within the E6-E104 range, the result in E107 remains the same as it was
prior to the change (and therefore no longer correct).
It does not recalculate the sum automatically.

What can I do to have this function recalculate automatically as soon as a
cell within the range changes (as it always has in the past)?

Thanks in advance...