Calculating cumative values

Hi,

I am working on a massive excel sheet and was wondering if there a simple way to do the following:

I have values such as -

A B C D

But I need to calculate (A*0) and in the next cell (A*1) + (B*0) and then in the next cell (A*2) + (B*1) + (C*0) and then (A*3) + (B+2) + (C*1) + (D*0) each in separate cells.

I can't just do the calculation and need to leave the cell reference for A, B etc. as these values will be changing.

At the moment I am doing it manually but there is a huge sheet and this is prone to human error.

Please let me know if there's an easier way.