ASP.NET

HttpContext.Current.Response.Clear();
HttpContext.Current.Response.ClearHeaders();
HttpContext.Current.Response.ClearContent();
HttpContext.Current.Response.Charset = "utf-8";
HttpContext.Current.Response.ContentEncoding = System.Text.Encoding.GetEncoding("Windows-1252");
HttpContext.Current.Response.AddHeader("content-disposition", String.Format("attachment; filename={0}", "\"File.xls\""));
HttpContext.Current.Response.ContentType = "application/ms-excel";

using (StringWriter sw = new StringWriter())
{
	using (HtmlTextWriter htw = new HtmlTextWriter(sw))
	{
		gridview.RenderControl(htw);
		
		HttpContext.Current.Response.Write(sw.ToString());
		HttpContext.Current.Response.Flush();
		HttpContext.Current.Response.End();
	}
}