You can populate a form using an ASPX page by adding controls to a container
(a Panel is a nice container, but even Page can be a container). If this is
what you mean by form (ie, a bunch of controls to fill in), you simply set up
a routine that dynamically creates controls and binds them to a container
(like a Panel, as mentioned above).

private void PaintForm()
{
   //assume proper reference to System.Web.UI.WebControls
  Label l = new Label();
  l.Text = "Name";
  Panel1.Controls.Add(l);

  TextBox t = new TextBox();
  t.ID = "txtName";
  t.Width = 50;
  t.MaxLength = 25;
  Panel1.Controls.Add(t);

  //Assume reference to System.Web.UI
  LiteralControl lc = new LiteralControl("<br>");
  Panel1.Controls.Add(lc);

  Button b = new Button();
  b.ID = "btnSubmit";
  b.Text = "Submit Name";

  //Add event handler  
  b.Click += new EventHandler(this.Button_Click);
}

NOTE: You will have to code in event handlers, like so:

private void Button_Click(object sender, EventArgs e)
{
}