//方法1：
// 创建MemeoryStream
   System.IO.MemoryStream ms = new System.IO.MemoryStream();
// 写dataset到MemeoryStream
   ds.WriteXml(ms,XmlWriteMode.IgnoreSchema);
Response.Clear();
// 下载附件的名字
   Response.AddHeader("Content-Disposition", "attachment; filename=Acounts.xml");
// 下载附件的大小，以便让浏览器显示进度条
   Response.AddHeader("Content-Length", ms.Length.ToString());
// 指定浏览器为下载模式
   Response.ContentType = "application/octet-stream";
// 发送到客户端
   byte[] b = ms.ToArray();
Response.OutputStream.Write(b,0,b.Length);
Response.End(); 

//方法2：

XmlTextWriter writer = new XmlTextWriter(Response.OutputStream, Encoding.UTF8);　　
writer.Formatting = Formatting.Indented;　　
writer.Indentation = 4;　
writer.IndentChar = ' ';　
writer.WriteRaw("<?xml version=\"1.0\" ?>");
ds.WriteXml(writer);
Response.AddHeader("Content-Disposition", "attachment; filename=Acounts.xml");
this.Response.ContentType = "text/xml";
Response.ContentType = "application/octet-stream";
writer.Flush();
Response.End();　　
writer.Close();