On a windows XP client that is a member of a domain, resetting the Windows time service to defaults (w32tm /unregister, w32tm /register) enables the NTP server on the XP machine itself.
(HKLM\System\CurrentControlSet\Services\W32Time\TimeProviders\NtpServer\Eanbled = 1)
It also says this in the MS docs on WTS registry settings here: http://technet.microsoft.com/en-us/library/cc773263(WS.10).aspx
The server shows as being enabled by doing a "netstat -a":
UDP XP-PC:ntp :
Is there any harm in disabling it?