How to avoid sorting in serial no iinside gridview

Hi

I am having datagridview in c#

I populated data inside datagridview.

I created Serial No inside datagridview along with other columns.

If i click serial no head,the serial no is sorted.

If i do sorting  in other columns,the serial no is also sorted.

How to avoid sorting serial no, if I click other column headers.

I have given the code below

             dsstock = new DataSet();
            dsstock = fetchDataSetValues(statement);
          //  grdStock.Rows.Clear();
            if (dsstock.Tables[0].Rows.Count > 0)
            {
                grdStock.Columns.Add("Sl. No.", "Sl. No.");
                grdStock.Columns[0].Width = 80;
                grdStock.Columns.Add("ItemID", "Item ID");
                grdStock.Columns[1].Width =300;
                grdStock.Columns.Add("ItemName", "Item Name");
                grdStock.Columns[2].Width = 600;
                grdStock.Columns.Add("ItemQuantity", "Item Quantity");
                grdStock.Columns[3].Width = 150;
                int row = dsstock.Tables[0].Rows.Count - 1;

                for (int r = 0; r <= row; r++)
                {
                    grdStock.Rows.Add();
                    grdStock.Rows[r].Cells[0].Value = r + 1;
                    grdStock.Rows[r].Cells[1].Value = dsstock.Tables[0].Rows[r].ItemArray[0];
                    grdStock.Rows[r].Cells[2].Value = dsstock.Tables[0].Rows[r].ItemArray[1];
                    grdStock.Rows[r].Cells[3].Value = dsstock.Tables[0].Rows[r].ItemArray[2];

                }
            }

Thanks

Chandran