Hi jcg,
An Alternative Solution could be :-
For the Even Rows ...
=SUM(IF(MOD(ROW(B2:B100),2)=0,B2:B100))
... and for the Odd Rows ...
=SUM(IF(MOD(ROW(B2:B100),2)=1,B2:B100))
Assuming your Data is in Cells B2:B100. Adjust to Suit your Ranges. Both Formulas should be Array Entered ( Ctrl+Shift+Enter ).
Hope this Helps.
All the Best.
SHADO