NTP Service in Windows

Ever wonder what is the Windows service which controls your NTP sync?

Windows Atomic Clock NTP Client Software and Windows Time Synchronisation Service.

TimeTools SNTP client software and the Windows Time Synchronisation service ('Windows Time') cannot co-exist on the same system, since they both attempt to use UDP port 123 (NTP). You can disable the Windows Time service from the service manager to avoid this problem and rely on the NTP time server monitor to synchronise time.

Operation with Windows XP SP2 Firewall

To operate any NTP or SNTP client on Windows XP SP2 or later, you must open the NTP port (Port 123) in the Windows XP firewall. From the Firewall Control Panel add UDP port 123 in the exceptions tab.

http://www.timetools.co.uk/time-servers/ntp-client.htm