Windows XP SP2 : The Death of my Chatting Application?

Question

hello, I made a simplest chatting application using UdpClient, it was working fine until I installed Win XP SP2. Now the messages are not being received at the other end. No Exceptions, nothing at all. I even disabled the Windows Firewall that comes with SP2, still no hope. My application is still working fine on loopback, but nothing after it. I am using port 20052. Can anyone explain what am I missing? Regards, Zishan