NetTime is a simple time synchronization client for Windows 95/98/NT/2000. Its main claim to fame is that it is small, simple and unobtrusive. And, of course, it's free software.

NIST :: Internet Time Service (ITS) allows users to synchronize computer clocks via the Internet. The time information provided by the service is directly traceable to UTC(NIST). The service responds to time requests from any Internet client in several formats including the DAYTIME, TIME, and NTP protocols.