can't play embedded youtube swf on some computers

I embedded a youtube video in my C# application like this :
string url = http://www.youtube.com/watch?v=x3r0ilu2Rzk;
StreamWriter sw = new StreamWriter("utube.html");
string PlayString = url.Trim().Replace("watch?v=", "v/");
string Finalplaycode = "";
sw.Write(Finalplaycode);
sw.Close();
string PathToNavigate = Directory.GetParent(Application.ExecutablePath) + @"\utube.html";
webBrowser1.Navigate(PathToNavigate);I can play it on my computer, but if I copy my dlls to someone else's computer, it won't work any more. It will only appears as a small red cross on the left top corner. We both have Adobe flash player installed. What is the problem?  Please help me.  Thank you!