With WindowsXP, you can double-click on the time in the System Tray and connect to Time Servers on the Internet to synchronize with.
To add to the list of available servers:
Go to HKEY_LOCAL_MACHINE\SOFTWARE\M'zoft\Windows\Curr entVersion\DateTime\Servers
Add new string keys named 2, 3, etc. with a value of the time server you want to use.