This is an old and annoying problem that likely happened to you when you installed 10.3 over 10.2.
However i think it really should have fixed itself by now with your upgrades to 10.4.
Give this a try:
Open Terminal (/Applications/Utilities/).
Type: sudo pico /Library/Preferences/com.apple.sharing.firewall.plist
Press Return.
Enter your administrator password when prompted, and press Return.
Using the arrow keys, scroll down to these two lines:
(key) allports (/key)
(array)
Directly under “(array)”, add a new line containing this text:
(string)515(/string)
Press Control-O to save the file, and Return to confirm.
Press Control-X to exit pico.
Restart the computer.