你所说的功能可以这样实现，但是你这样加密无用。只要通过将DOC文件拖动到WORD只照样可以打开
public static void FileRelating(string apppath, string appname)
        {
            try
            {
                Registry.ClassesRoot.CreateSubKey(".doc");
                RegistryKey sgp = Registry.ClassesRoot.OpenSubKey(".doc", true);
                sgp.SetValue("", "docfile");
                sgp.Close();
                Registry.ClassesRoot.CreateSubKey("docfile");
                RegistryKey sgp2 = Registry.ClassesRoot.OpenSubKey("docfile", true);
                sgp2.CreateSubKey("DefaultIcon");
                sgp2.CreateSubKey("shell");
                sgp2.Close();
                RegistryKey sgp3 = Registry.ClassesRoot.OpenSubKey("docfile\\DefaultIcon", true);
                sgp3.SetValue("", apppath);
                sgp3.Close();
                RegistryKey sgp4 = Registry.ClassesRoot.OpenSubKey("docfile\\shell", true);
                sgp4.CreateSubKey("解密");
                sgp4.Close();
                RegistryKey sgp5 = Registry.ClassesRoot.OpenSubKey("docfile\\shell\\解密", true);
                sgp5.CreateSubKey("command");
                sgp5.Close();
                RegistryKey sgp6 = Registry.ClassesRoot.OpenSubKey("docfile\\shell\\解密\\command", true);
                sgp6.SetValue("", appname + " \\F %1");
                sgp6.Close();
            }
            catch
            {
            }
        }

http://down1.tomore.com/source/20071008/2007100821513027705.rar

这个是我用C#做的一个加密程序。如果想文件加密可以看看这个小程序。