To sum up random cells in Excel 2003, use the following steps:
[list=1][*]Select the cell where you want the sum to appear.[*]Type in the formula bar.[*]Click on the first cell you want to include in the sum. For example, A2.[*]Type a plus sign (+) to add the next cell to the sum.[*]Click on the next cell you want to include in the sum. For example, A7.[*]Repeat steps 4 and 5 for each additional cell you want to include in the sum. For example, +C4+D7+G3.[*]Type a closing parenthesis ()) to complete the formula.[*]Press Enter to calculate the sum.

Your formula should look something like this: