http://www.woodwardweb.com/dotnet/generating_exce.html

Response.Clear();
Response.Charset = "";

// set MIME type to be Excel file.
Response.ContentType = "application/vnd.ms-excel";

// add a header to response to force download (specifying filename)
Response.AddHeader("Content-Disposition", "attachment; filename=\"MyFile.xls\"");

// Send the data. Tab delimited, with newlines.
Response.Write("Col1\tCol2\tCol3\tCol4\n");

Response.Write("Data 1\tData 2\tData 3\tData 4\n");
Response.Write("Data 1\tData 2\tData 3\tData 4\n");
Response.Write("Data 1\tData 2\tData 3\tData 4\n");
Response.Write("Data 1\tData 2\tData 3\tData 4\n");

// Close response stream.
Response.End();