Couldn't use and aggregate funtion on an another aggregate function

I have a column that has an expression with a runningvalue in it, a sum of "PO Count" for each month. I need to create another column that aggregates the SUM of "PO Count %". I can't to do a Runningvalue on the Runingvalue. I can't even do a Sum on the Runningvalue.

Below is a message:
The value expression for the textbox 'textbox4' contains an aggregate function (or RunningValue or RowNumber functions) in the argument to another aggregate function (or RunningValue). Aggregate functions cannot be nested inside other aggregate functions.

I am using stored procedure and whnever I try to add a calculated field, Visual studio itself is getting hanged and asked me to close.

I am using Ms-SQL server 2005 and Vs 2005 Team System Enterprise Architect editions.

Any help?

Thanks in indavnce

Visualcpp

Aggregates of aggregates are not supported