The default or official gnutella protocol is port 6346 & uses both TCP & UDP but mostly only one of them. I can't recall which off-hand. Is that what you wanted to know? Gnutella Protocol

You're the 2nd person who has upgraded to xp media centre & is now having problems. At least it's similar. http://www.gnutellaforums.com/showthread.php?t=63933

This might or might not be helpful MS & 3rd party patches for SP2