Highlight the whole column, right click and go to "format". A window will open up and you can set the value of the "numbers". You can change it to add or subtract. You can change decimals. And you can change how the sums are displayed.
Now, it's been about 4 years since I played with Excel, but I do remember that you have to change the default when you're working with stuff like this.
Hope this helps a little.