connection string and backup operation

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C#3.5

Hi,
I am a programmer Italian, I do not know English well. I hope to explain my problem well.

I develop in C # on a PC with Windows Seven Pro, vs 2010 pro, sql server 2008.

I have problems with the backup operation and the connection string.

The code is as follows:

private void BK()
        {

            string strconn = @"Data Source=.\SQLEXPRESS; AttachDbFilename=|DataDirectory|\db.mdf;Integrated Security=True;User Instance=True"; 

            SqlConnection conn = new SqlConnection();

            conn.ConnectionString = strconn;

            try
            {

                //Query per backup

                string queryBK = "BACKUP DATABASE db TO DISK ='C:\\Program Files\\Microsoft SQLServer\\MSSQL10.SQLEXPRESS\\MSSQL\\Backup\\db.bak' WITH INIT, SKIP, CHECKSUM";

                //Creazione Command   
          
                SqlCommand cmdBK = new SqlCommand(queryBK, conn);

                // Open connection.

                conn.Open();

                //Execute command     
            
                cmdBK.ExecuteNonQuery();

                MessageBox.Show("backup effettuato");
            }
            catch (Exception ex)
            {
                // Process exception.

                MessageBox.Show(ex.Message, "ERRORE", MessageBoxButtons.OK, MessageBoxIcon.Error);
            }
            finally
            {
                conn.Close();
            }
        }

This code works on the development PC, but if I install my application on another PC (with Vista) does not work, returned the following error:

"The database does not exist. Verify that the name has been entered correctly. INTERRUPTION
ANOMALOUS BACKUP DATABASE."

I would stress that this string works well with the operations INSERT, DELETE, UPDATE
on both my PC and on the PC test.

If I replace the connection string with:

string strconn = @"Data Source=.\SQLEXPRESS; Database = db;Trusted_Connection =True";

The string work on my PC but on my test computer returns the following error:

"Can not open database requested by the login. Login failed.
Login failed for user Pina-PC \ Pina "

Someone could help me to solve this problem?
Thanks...Iole