I need to sum values by month/year (Ex: Jan 2006) based on data that I've set
up in an Excel spreadsheet.

My spreadsheet is set up in this fashion.
StartMonth : StartYear: Month1 : Month2 : Month3 : ...Month24
12 : 2005: 400 : 250 : 1000 : etc
1 : 2006 : 300 : 650 : 2500 : etc

Based on the Start Month and Start Year values, I want to sum the
appropriate figures into a Grand total for that particular month.

In this instance, Jan 2006 should = 550, Feb 2006=1650, Mar 2006=2500

Does anyone have a solution to this? I'm in desperate need of some help.
Thank you.