Up to Windows 2000, using an NTP server as the time source was as simple as entering the server address in the Time/Date control panel. From 2003 onwards, you must follow the procedure detailed in KB816042 to configure the Windows Time service to use an external time source.

Paraphrasing:

• HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\W32Time\Parameters\Type=”NTP”
• HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\W32Time\Config\AnnounceFlags=5
• HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\W32Time\TimeProviders\NtpServer=1
• HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\W32Time\Parameters=”pool.ntp.org,0×1″
• HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\W32Time\TimeProviders\NtpClient\SpecialPollInterval=900
• HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\W32Time\Config\MaxPosPhaseCorrection=3600
• HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\W32Time\Config\MaxNegPhaseCorrection =3600
• net stop w32time && net start w32time

You may replace pool.ntp.org above for any other server (or servers, space-separated) with “,0×1″ after each host name. And keep an eye in the Event Log after you restart the service.