Windows XP allows you to select your preferred time server from a list found on the Internet Time tab in the Time and Date program. Although this list includes a number of servers, it’s also possible to configure XP to use a time server a little closer to home by editing the Registry. A list of SNTP servers in located in the Registry under HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\Current Version\DateTime\Servers.
You can add additional servers to this list by creating additional values of type REG_SZ, using the next available number on the list. Once added, this time servers will be available for selection on the Internet Time tab. For a list of public SNTP servers in the UK and elsewhere, see http://support.microsoft.com/?kbid=262680. As a general rule, only second-level time servers allow public access.