Since Service Pack 2 for windows XP was released, Windows contains a firewall feature that may interfere with the normal operation of Time synchronization products.
This describes how to configure the firewall to allow unsolicited NTP messages through to K9 and Tardis.
Open the firewall control panel. Make sure the settings are set like this
On the Exception tab click on 'Add Port'
Then add a setting to allow NTP.
Press OK, and everything should work like it did before SP2 improved things.