void RenameThem()
{
    DirectoryInfo d = new DirectoryInfo("c:/dir/");
    FileInfo[] infos = d.GetFiles("*.myfiles");
    foreach(FileInfo f in infos)
    {
        // Do the renaming here
        File.Move(f.FullName, Path.Combine(f.Directory.ToString(), "1" + f.Name));
    }
}