Errata....

I wrote:

=SUMPRODUCT((A1:A100="xyz")*(B1:B100=""),C1:C100)

This form is easily extended to handle OR (replace "*" with "+").

Wrong! That would not work if both conditions are true.

So how would we handle the OR of the two conditions using SUMPRODUCT.
Sigh, I am much too tired after driving for 5 hours after a couple of
sleepless nights.