when I autofill and save excel becomes non responsive

I am having this issue with excel 2007... I have a spreadsheet that I am trying to use auto fill on... everytime i drag it down and hit save the program quits responding... I AM SO FRUSTRATED!. I am sure that there is an easier way, but I don't know it. Here is my formula that I am dragging down whenever I fill in the other cells.

=D12+B13-C13

so the items in column B are charges and in C are credits and D is the total... so when I type something in B or C I drag down the D cells so it does the math.

If I just type something in the sheet and not use autofill it will save... please help

THANKS IN ADVANCE