Wayne and Dave's answers show you how to open a word document in code, however if you just want to open the document in word try something like
using System.Diagnostics;
public class WordHelper
{
public static void OpenMicrosoftWord(string filePath)
{
ProcessStartInfo startInfo = new ProcessStartInfo();
startInfo.FileName = "WINWORD.EXE";
startInfo.Arguments = filePath;
Process.Start(startInfo);
}
}