XP Pro; Excel 2003

Here's what I'm trying to do.

if A1:A10 and left (B1:B10,2) = D1 & E1 then sum C1:C10

Here's my formula = SUMIF(A1:A10 & LEFT(B1:B10,2),D1&E1,C1:C10)

Can't get it to take, either as a formula or as an array.

I may need to use sumproduct. I would need help with sumproduct as I'm still learning to use sumproduct.

I would rather not add any more helper columns, but I know I can concatenate this data in the helper columns and then use a simple sumif formula with a single condition for the first argument.

Any thoughts?

THANK YOU.