if you're in the US, try time.nist.gov

The Windows Time Service in Windows 2000/2003/XP is also based on NTP.
Only the server variants operate as NTP servers, though. So if you have a Windows 2000/2003 Active Directory Domain Controller in your network, you can use that as an NTP server, without having to configure anything.

You may, however, prefer to sync this server itself to a reliable NTP source using 

net time /setsntp:your.real.ntp.server


Pim