В операции должен использоваться обновляемый запрос

void cmdOk_Click(Object sender, EventArgs e) {
      string conn_str = 'PROVIDER=MICROSOFT.JET.OLEDB.4.0;DATA SOURCE='+Server.MapPath('gbook.mdb');
      OleDbConnection conn = new OleDbConnection(conn_str);
      OleDbCommand cmd = new OleDbCommand('SELECT * FROM gbook', conn);
      OleDbDataAdapter adapter=new OleDbDataAdapter(cmd);
      DataSet gb=new DataSet();
      try{
      conn.Open();
      adapter.FillSchema(gb,SchemaType.Mapped,'gbook'); }
      catch (Exception err){Response.Write(err.Message);}
      finally {if (conn!=null)conn.Close();}
      //добавление данных
      DataRow rowNew;
      rowNew=gb.Tables['gbook'].NewRow();
      rowNew['name']=EncodeData(name.Text);
      if(mail.Text!='') rowNew['email']='mailto:'+EncodeData(mail.Text);else rowNew['email']='';
      string site=url.Text;
      if ((url.Text.ToUpper().Substring(0,7)!='HTTP://')&&(url.Text!='')) site = 'http://' + site;
      rowNew['http']=EncodeData(site);
      rowNew['data']=DateTime.Now;
      rowNew['body']=EncodeData(comm.Text);
      gb.Tables['gbook'].Rows.Add(rowNew);
      int added=0;
      try {
      OleDbCommandBuilder cb= new OleDbCommandBuilder(adapter);
      adapter=cb.DataAdapter;
      conn.Open();
      added=adapter.Update(gb,'gbook');
      }
      catch (Exception err){Response.Write(err.Message);}
      finally {if (conn!=null)conn.Close();}
      if (added>0){
      GetDataSet();
      DG.DataBind();
      HttpCookie cookie=new HttpCookie('ASPNETGBOOK');
      cookie['name']=EncodeData(name.Text);
      cookie['email']=EncodeData(mail.Text);
      cookie['http']=EncodeData(site);
      Response.Cookies.Add(cookie);
      cookie.Expires=DateTime.Now.AddYears(1);
      }
    
    }

OleDbConnection conn = new OleDbConnection(conn_str);
conn.Mode=3;