Internet Explorer object "Object reference not set to an instance of an object."

I've been trying to write an aplication to automatically log me in to facebook, pointless i know but it's a step to a greater goal. Anyway, here's the sticking point:
 
using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;
using SHDocVw;

namespace SpamThis
{
    public partial class Form1 : Form
    {
        string UserNames = "";
        string Passwords = "";
        InternetExplorer Browser;
        HtmlDocument Doc;

        public Form1()
        {
            InitializeComponent();
        }

        private void UserName_TextChanged(object sender, EventArgs e)
        {
            UserNames = UserName.Text;
        }

        private void Password_TextChanged(object sender, EventArgs e)
        {
            Passwords = Password.Text;
        }

        private void Login_Click(object sender, EventArgs e)
        {
            Browser = new InternetExplorer();
            object o = null;
            Browser.Visible = true;
            Browser.AddressBar = false;
            Browser.Navigate("http://www.facebook.com/", ref o, ref o, ref o, ref o);
            while (Browser.Busy)
            {
            }
            Doc.GetElementById("reg_email__").SetAttribute("reg_email__", UserNames);
        }
    }
}

The line highlighted in red gives me this error: "Object reference not set to an instance of an object."
And then when i try adding this line just above it: Doc = (HtmlDocument)Browser.Document;
it says that i can not convert to this type. How do i retrieve and edit the instance of IE's html document?
Is this the correct way to implement this code? Can anyone help?

Much appreciated