Summing two columns in a new column

[Gerhard]

H

I have two columns, A and B. I want to have a third column, C, by multiplying column A and B in an inverse order. That is to mean C1=A1*B1, C2=A2*B1+A1*B2, C3=A3*B1+A2*B2+A1*B3, C4=A4*B1+A3*B2+A2*B3+A1*B4, etc. Is there any way through which I can accomplish this job easily, for example by using SUMPRODUCT, etc. A VBA script is also welcome. I am using Office XP

Thanks