Windows 2000 and XP already have built in NTP time synchronization abilities.
Check out this pdf for how to configure Windows 2000 and XP to use the NIST time servers:
http://tf.nist.gov/timefreq/service/pdf/win2000xp.pdf
Instructions from Microsoft on how to change the update interval of the built in time service:
http://support.microsoft.com/kb/q223184/