Unable to load SSRS reports .

Hi All,
I have created one Rdl file & hosted in report server.I am able to access the Rdl hosted server link & generate the report in clinet side (Local IE 8 browser).
But When I use in .Net application , Getting the error :"The request failed with HTTP status 401: Unauthorized"

ReportExecutionService rs = new ReportExecutionService();
execInfo = rs.LoadReport(reportPath, historyID); --- In this line i am getting the above error.

In Browser , i am able to access the server link like : http://IPaddress/Reports - In this line able to generate the report by passing the required parameters.

But in application I have configured like :
<!--URL to the report server to access SSRS reports-->
<add key="ReportServerURL" value="http://IPaddress/Reportserver"/>
<!-- URL to the report server to access SSRS reports -->
<add key="ReportPath" value="/SampleReports"/>

Please help me to resolve this error.

Thanks,
Nagababu.