Here is a sample file with the running total formulas in the colored cells. Please note that the first cell on top (colored orange) has a different formula than the rest. So if you need to fill the formula down to the cells bellow make sure you start from a yellow cell.
Hope this helps. Let me know if you have any questions.
Ok, that can only be done by code since the formula in C2 cannot add values to itself. I have just written the code for this using Excel standard VBA programming language. However I noticed that you are running Excel 2008 which unfortunately no longer supports VBA. If you have Excel 2004 or any Excel for Windows I will be happy to send you the file with the code.
Let me know.