Go to Start/Run, and start a DOS box with CMD (for NT/2000/XP machines) or COMMAND (95/98 machines)

At the command prompt, just type NET TIME and it should tell you what the time is and the name of the time server you are pointing at. That should allow you to carry out a little detective work on your network to find out which machines are time servers (running NTP or some such). When you know which ones they are, then you should be able to set them correctly. There should really be only one time server in a LAN bounded by a router as the gateway.

To check if your machine is running an NTP client to listen to time change broadcasts, from the prompt type NETSTAT /a and look for a socket on your own machine listing to the NTP service somewhere on your LAN, identified by (I think) port number 123 (NTP).