Ssrs 2005

i can show the Report in a winform using Reportviewer Control
thats good, but this for only the user "sa" pwd "***"
i want to change the DataSource Credentials in runtime can this be done

i try

Code:

---

ReportingService2005 rs;

// if i put this line it works fine
//rs.Credentials = System.Net.CredentialCache.DefaultCredentials;
// if i add this line it gives me error
rs.Credentials = new System.Net.NetworkCredential("aaa", "412");
                       
           
                        bool forRendering = true;
                        string historyID = null;
                        ParameterValue[] values = null;
            DataSourceCredentials[] credentials = null;
                        ReportParameter[] parametersSSRS = null;
                        ValidValue[] pvs = null;

parametersSSRS = rs.GetReportParameters(report, historyID, forRendering, values, credentials);

---

and here the code stop
it gives me erorr

The request failed with HTTP status 401: Access Denied.

is it hard or iam posting the Question in the wrong section?