With XP, why not use the built-in feature for synchronization of time? To do this, go to Control Panel, Date and Time Properties, select the Internet Time tab, and check the box for "Automatically synchronize with an Internet time server".
I don't know whether Win 2000 and Server 2003 have the same feature. You always Google on Internet time synchronization; I believe there's plenty of info and tools already available for this.