Tak zkus:

String sConnectionString = "Provider=Microsoft.Jet.OLEDB.4.0; Data Source=" + Server.MapPath("data.xls") + ";Extended Properties=Excel 8.0;HDR=No;IMEX=1;";

String sConnectionString = "Provider=Microsoft.Jet.OLEDB.4.0; Data Source=" + Server.MapPath("data.xls") + ";Extended Properties=Excel 8.0;HDR=No;";