Note to Jo:
In Excel 2000 anyway, it must be a list or a database, so you cannot
start the data on row 1. The hidden rows must be the result of filtering
the rows.

Note to all:
In Excel 2000, therefore, the formula for SUM would be (note the
different function number):
=SUBTOTAL(9,A2:A5)

and the formula for AVERAGE would be (again Excel 2000):
=SUBTOTAL(1,A2:A5)

I guess Microsoft must have expanded the function numbers from Excel
2000 to Excel 2007! They must have an import wizard to convert older
worksheets to newer ones upon opening them!
--
Regards,
Bill Renaud