Summing Values From Array W/ Multiple Conditions-cols

I know there have been many posts about multiple conditions in arrays, but I didn't find one that applied to my situation, so I'm hoping can help me out of jam. I'm using Excel 2000 on XP.

I have a database of sales information. Customer name is listed in column C, and columns F through Q contain the sales for the months of July - June (fiscal calendar year), with the headers for the months in row 4. There can be many rows of sales for the same customer, hence the need to sum them.

Example

C F G H
CUSTOMER JULY AUGUST SEPTEMBER
customer1 2 3 4
customer1 2 3 4
customer2 2 3 4
customer2 2 3 4

The problem is that this information will need to be updated every month to reflect sales year-to-date, meaning that the conditions for summing the data will change depending on how far into the year we are. So for instance, in August I will need to sum all the records for customer1 in the July column (column F) and the August column (column G), but next month it will have to sum the records for customer1 from July, August, and September (col H).

The result of this information would be displayed on a different worksheet. My thought was to assign a range of 12 cells (Sheet2!A1:A12) and to populate those cells with any of the months that need to be summed thus far. So for example, through September, A1 = "July", A2 = "August", and A3 = "September". Then the formula would use those values to evaulate the conditions. So in plain English the formula would:

sum the values in columns F : Q where the monthly header = the values in the range Sheet2!A1:A12 for all records where CUSTOMER = CustomerName