In Your aspx Page you.
(1).Import
using System.IO;
using System.Text;
(2).Use Server.Execute method.
protected void Page_Load(object sender, EventArgs e)
{
StringWriter strWriter = new StringWriter();
Server.Execute("~/Login.aspx", strWriter, false);
string str = strWriter.ToString();
}
variable str will contains Whole Html Output of Login.aspx page.
Thanks..