If you mean, running your XP box as NTP server, then you would need to get a
hold of some 3rd party Time Service that can be run on an XP box.
If you mean, how can I tell my XP box what his local NTP server is, then you
need to use the NET TIME command, eg.
NET TIME \\yourcomputername /SETSNTP[:ntp server list]
Checfk out the description in Help and Support.