We can use the files concept to load the doc file content
into the textbox using the Multiline property.

we have many ways.This is one of the way.

using System.IO;


FileStream fstream = new
FileStream("Sample.doc",FileMode.Open,FileAccess.Read);
StreamReader sreader = new StreamReader(fstream);

txtFileContent.Text = sreader.ReadToEnd();


->change your Text Box Property into tghe multilien txtbox.

we have to add referance of word document from COM 
referance..
because it only work with .txt files..

ApplicationClass WordOper = new ApplicationClass();
String filePath = Server.MapPath("../Files/Book.doc");
Object file = filePath;
object nullobj = System.Reflection.Missing.Value;
Document doc = WordOper.Documents.Open(ref file, ref
nullobj, ref nullobj,
ref nullobj, ref nullobj, ref nullobj, ref nullobj, ref nullobj,
ref nullobj, ref nullobj, ref nullobj, ref nullobj,
ref nullobj, ref nullobj, ref nullobj, ref nullobj);
Document doc1 = WordOper.ActiveDocument;
string m_Content = doc1.Content.Text;
TextBox1.Text = m_Content;
doc.Close(ref nullobj, ref nullobj, ref nullobj);